HI Steve, welcome!
In the early '70's Westerly operated much like a custom shop anyway, in that they would build one-offs and short runs of special builds without necessarily having made them to an order.
That would be as opposed to "special order" models like the D55 which was introduced as special order only in '68, finally becoming a regular production model in '74.
The first Guild 12's were the F212 (16" lower bout) and F312 ca late '64-'65. Those early models frequently featured extra-wide (nearly 2") nut widths and flat radius boards. Ralf towner (edit: corrected from Pat Metheny, had a senior moment) was a fan of that variant.
Next up in '68 was F412 based on the F50 (arched maple backs 17" lower bout). The F412 begat the very earliest F512's as "F412
Specials", which, as far as I can tell, were built "just to try it". IIRC early F412's/512's were special order only, and that being the case, easily tailored to a customer's special requests (within taste and reason.)
("Special" usually designated an instrument that was built with a significant spec variant, in this case, the F412 got a rosewood body.)
In the case of the D55, Tommy Smothers
ordered D50's with F50 necks (bling) and these were originally labeled "D50 Special" before they finally made it a "standard model".
So with the F512's things got really interesting. The detail I remember most clearly is the ones that got special fretboard inlays, nice review here in Vintage Guitar:
https://www.vintageguitar.com/29226/guild-f-512/
The other exotic variant I recall were the
arched rosewood back F512's. These were based on the the same treatment given to some late '60's F50's of which John Denver and Bob Weir are known owners.
The curious detail about these backs was that although they're truly laminated "one-piece" arched backs, they look like like traditional book-matched 2-sided (2 piece
solid) backs. IIRC part of the problem was simply obtaining rosewood veneer of sufficient size to make a one piece (laminated) arched back, so Guild simply used 2 pieces of rosewood laminated to a center sheet(s) on both sides, and even put a marquetry stripe down the center which
really throws some people off.
So they take that glue-laminated sheet and put it in a steam press with the appropriate size archback dies installed and press the back into shape. It's really a quite elegant and durable construction. But it's possible an arched back might create sonic overkill with rosewood, that might explain why the configuration is so rare in general.
So I doubt there were more than 6 of 'em ever built, that was Guild's standard "batch size". I wouldn't even know about the archback 512's except a guy with a matching pair of the F50/F512 archbacks joined and posted few years back.
Others might have more interesting details to share.
So are you in the hunt?