In the pic of the back, which shows part of the headstock, you can see a mechanical device attached near/in place of the low-E tuner. I presume this device's function is to allow the player to quickly switch between regular and "drop D" tuning, by just moving the lever that is visible.
Yup.
The Drop D tuner is a gizmo that appeared in the 1970s.
It's an eccentric cam. The thumb tab controls it. The cam goes next to the string before the string is inserted into the tuning peg.
The guitar string is tuned with the at it's widest spot ben ding the string to the high note, E. When the tab is pushed downward, the cam releases the string a whole step down to D.
They're actually a pretty simple device that's sturdy. Hipshot is the company that makes them. They have several different ones that aren't designed to go on a peghead. I don't know if Hipshot is still in business or not, but they weren't a large company.
A buddy of mine helped design it, and he had several of them on some of his guitars. They were mostly used on electric guitars, particularly the Telecaster, and I've never heard of one breaking. I think the one on that guitar must be simply out of proper position or needs some other adjustment.
They work well, but would be harder to operate on a heavy acoustic bass string than on the electric's lighter gauged bass string. The Telecaster already had a device that could drop the 3rd string a full tone and then raise it back to pitch, so the Tele was a natural for this one. They can be used in performance, and give the Tele some steel-guitar effects.
Banjos have a similar devices that work as a pair that's similar, but can change the pitch of both the 2nd and 3rd strings to give the banjo steel-guitar effects. Steel guitars use knee and foot levers. The pitch raises and lowers smoothly and stops at the correct note in either direction. The best designs can be tuned to any 2 pitches, up or down, and will always stop at the right pitch.
A conventional tuning peg can be used by ear if the peg has a short ratio that doesn't demand many turns of the shaft. The precise note is harder to catch by ear alone though, so the technique is seldom done with conventional tuning pegs.
Earl Scruggs invented the first version, which requires 2 extra tuning pegs (and 2 more holes in the peghead) to operate. They allow the banjo to drop from an open G tuning to an open D tuning while playing.
Scruggs' version worked very well, but they sure messed up the looks of a banjo's peghead, which are typically covered with pearl inlays.
(Earl thought his looked so ugly he covered them with a piece of time he cut from his wife's floor waxer.)
And on some peg heads that are cut in an ornamental shape that often includes a narrow waist cut, they won't work at all. Most banjo peg heads have a figure 8 shape or something even more ornate. It's part of banjo tradition.
About 10 years after Scruggs invented them, another killer banjoist, Bill Keith, invented a new banjo tuning peg that had the D-tuner built inside the gearbox of the peg. These tuners could be used conventionally, like any other good tuning peg, but when set with some tumbscrews on the outside of the gearbox, the peg could be set at any tuning up or down, the player wanted.
These Keith pegs look like conventional vertical banjo tuning pegs from the front, and almost the same from the back. Some Telecaster players adopted one of them for a bass string tuner.
The Keiths were always the most expensive tuning peg on the market. Back in the 60s, a set of Grovers, good planetary tuners, cost $20 for a set of 4. The Keiths cost $50 for two. Equally good vertical tuners mounted on a plate, the kind used on classical guitars, cost $12.
If a banjo player wanted a high-ratio tuning peg, normally 12:1 ratio, the only peg that worked was the Grover Rotomatic. Most banjoists prefer the short ratio still, mostly because the 5-string banjo has so many different tunings. 4:1 changes a string's pitch a lot faster than 12:1.
By design, all the vertical tuners have always had a much smaller gear ratio than the side-geared guitar tuners. A typical banjo tuner has a 4:1 ratio, or lower- 2:1 or even 1:1. Their vertical design allows them to be used on any banjo peghead, and they are easier to grab, as banjos all have a longer scale than guitars.
Just a couple of years ago, this was it. If a player wanted a slower ratio, a sidewinder like the guitar peg was the only way to get it.
But now, there's an entirely different gearing design that's on the market that has never been used before.
Here's the story:
Around 2020, a Canadian guy named Bill Rickard who's a mechanical engineer, miniaturized an entirely different gearing system Frank Ford invented. This system uses cyclonic action to turn the shaft, a very old design that has been used on heavy equipment for a long time.
The cyclone tuner has some big advantages: the peg isn't directly connected to the shaft, so a vertical tuner doesn't have to be set tightly to hold pitch. The peg can be very easy to turn, not so tight as the planet design demands.
The other big advantage is it allows a 10:1 ratio. That's slightly faster than a guitar tuner, but not much slower than the quick-tuning planetary tuner.
They're superb tuning machines, and are costly; a set of 4 costs $200. So far, only the banjo community knows about them, but they would work as well on a guitar as on a banjo for players who use alternate tunings a lot.
That's not the most fascinating part of the story.
Bill is a very singular guy. I've known him for years on the internet as a fellow member of the Banjo Hangout.
Bill first came onto the Hangout to ask a question:
Is there any way a person who only has one arm can play the 5-string banjo?
Bill Rickard had played the 5-string banjo, a dedicated fingerstyle instrument, all his life. Until his 50s.
While he was on a vacation in England, he was riding a motorcycle that collided with a delivery truck. The doctors had to amputate his right arm at the shoulder and his right leg at the hip to keep him alive. Even his shoulder and hip sockets are part way gone.
So the answer that came from the sympathetic members of the Hangout eventually became No.
There was no way he could ever play the banjo again.
That was the end of Bill on the Hangout for the next 3 years. We all thought he would never return.
Once he was finally fully recuperated and accepted his playing days were over, Bill went back to work as an industrial designer. If he couldn't play the banjo any more, he decided could still make banjo parts, so he set about trying to make them. At first, as part of his mental and physical rehabilitation.
He invented an entire workshop he could use in a wheelchair down in his basement, and set about working, all by himself, on making banjo parts. All his workbenches have clamps, servo motors, etc. to replace his missing limbs. Many are those he designed and made himself, though others are standard hardware store stuff.
His wheelchair is motorized, and equipped with hand and foot controls that all plug into his servo stuff on his benches. He's doing saw work, drilling, milling, spinning brass, turning, and polishing, assembly and setup with one hand and one foot.
And his parts are some of the very best that can be made. From the first he offered for sale, they were all top quality.
He's one of only three banjo makers that make all their parts in-house. (except for fret wire. That's a specialized part of the wire industry.)
Eventually, he found a good young apprentice to help him in production, and nowadays Bill is making entire banjos, and producing enough parts his little business is both viable and profitable.
Frank Ford gave Bill is cyclone tuning pegs, so large they can only work on an electric bass, to Bill just to see if Bill could miniaturize them down to the size of a standard planetary tuner. Bill worked on it off and on for years, but he did it. Frank had given up trying by then and wasn't sure the parts could be smaller than his. It was a big challenge for them both.
The Cyclones are the smoothest tuning machines I've ever used, and they hold their pitch better than all others. They are no larger than a conventional Planet, and they even look elegant. They have a graceful curve in the gearbox, which is polished brass. All the buttons are polished hardwoods, though he began using some plastic ivoroid buttons he had made to his specifications due to buyer demand.
The guy is amazing human being. Even the boxes he uses are ones he makes. They all have magnetic latches, and all the lettering is milled into the box surface; no paper labels or printing at all on the box. A supremely good little stash box that will last forever.
They're a drop-in replacement for Grovers, 5-Stars, Gotohs, etc., so they should work on a guitar that has any of those brands, and would be especially good replacements for the wooden friction pegs used on flamenco guitars. Any guitar with a solid peghead would have a drop-in fit, but not on a guitar with a slotted peghead.
Looks are the least important, but it has to smell nice. I just read a thread on AGF about a guitar purchase off Reverb where the heavy smoke smell was not disclosed. That would be a non-starter. I'd rather buy something that I've played personally vs. ordering online, but it might not be possible. 
