Interesting Dave. So Colosi's Martin "drop in" saddles are a dead ringer for a Guild fit?
West
I wouldn't call Colosi's saddles a "dead ringer," but fitting them to a Guild is pretty easy. I used them frequently before I started making my own saddles from blanks. I liked Bob's saddles... they were consistently high quality, and (IMO) the top radius was sufficiently close to that of Guilds fret boards that I didn't need to make any corrections. Bob sends good written instructions with pictures along with each saddle... the only step that isn't covered is trimming to length to fit the Guild slot.
Colosi's 3/32" Martin drop in saddle is a usually just a smidge too thick to fit into a Westerly Guild saddle slot... so just a little sanding is required to thin it down. You want it to slide easily into the slot without forcing, but be sufficiently tight that it won't fall out if you turn the guitar face down. Just a little sanding of the top is then needed to round off any sharp edge (IIRC, Bob covers this in his instructions).
The saddle slot on all my Westerly-era dreads was shorter than the Colosi saddles, requiring that they be cut to length. The only tricky part was making the cuts so that compensation aligned correctly with the B (2nd) string. IIRC, you end up cutting a little (but not equal amounts?) from BOTH ENDS. Then I round and smooth the ends to fit the rounded ends of the Guild saddle slot, and round the top corners (for aesthetics).
In my experience, the most time-consuming step is sanding to the proper height... you just have to go slow and keep checking the string action with the saddle installed. I was always replacing an existing saddle, with the intent of either raising or lowering the action... using the old saddle as a guide, I would mark the bottom of the saddle with a line that would leave the saddle just a little taller than I wanted, sand the bottom carefully to that line (Bob gives instructions and sanding in a manner to keep the bottom flat), install and check action, decide how much more needs to come off... sand a little more, install and check the action... etc. It might take 3 or 4 iterations to finally get the height I wanted.
Because my process required multiple checks with the strings brought to tension, I would usually try and do the saddle fitting with OLD strings, and install new strings AFTER I had made all the adjustments. The first one I ever did probably took me close to 2 hours... much of that resulting from "Measure twice, cut once!" mentality. Later, I was able to fit a saddle in about 45 min.
My worst experience was fitting the saddle to my DV-72MK during it's initial set up shortly after I bought it. By this time, I had done the procedure many times...I measured carefully against the old saddle, sanded to the line... and IT WAS TOO SHORT! I must have measured wrong, so I got out another Colosi saddle I had, went thru the whole process again, and IT WAS TOO SHORT AGAIN! After much bad language (I had just "wasted**" two Colosi saddles) and head scratching, I finally realized that the old saddle I was replacing, and that I was measuring against, had not been fitted properly. When installed, the old saddle did not reach the bottom of the saddle slot, but was wedging in the slot part way down! (which is a potential danger of splitting the bridge!). Because I was now out of Colosi saddles, I had to make one from a blank I had...
**I was able to use one of the "too short" Colosi saddles later on the D-70 I bought from BeeCee. I still have the other, and might be able to use it in the future on another guitar.
I haven't had to make/adjust/install a saddle for a couple years now (since I boght my D-70 from BeeCee in 2020)... but think I will need to soon, the saddle on my DV-73 is starting to show some wear notches along its top from the strings. I'll have to rummage thru my guitar tool box to see if I have any bone blanks, or maybe I'll just take the short cut and order a saddle from Bob Colosi...
