So I did end up purchasing the guitar and have been doing a bunch of close inspection. I can only guess that the guitar must have been refinished by Gibson between 1933 and 1943 when the current logo was in use. I DID find the Factory Order Number which is very faint, but present on the neck block and is absolutely correct for 1906.
It is a nitro finish with checking, which is definitely not original, but really rather old. I would have thought the bridge should be ebony, but who knows... it looks extremely old too, and is a perfect match in shape for what an original would look like.
There doesn't appear to be any major structural repairs, and while it's difficult to say for sure the frets appear original.
I have found *some* pictures of early 1900s Model O and L-1 /L-3 guitars with this type of bridge etc.
Body depth, bridge type, rosette type, neck join etc all are consistent with 1906. There is also no truss rod which means the neck is likely original and definitely pre-1921.
Lastly, there is a portion of the kerfing on the back, roughly 2 1/2 inches long, right next to the treble side of the neck block that appears to be replaced, and I'll tell you. Whoever did that, did one helluva professional job, and it was done LONG ago. So I'm assuming that was done also in the 30's.
Regarding the action and possibility of a neck reset.
Having ummed and ahhed about this for a while, I decided to shave the bridge down. The bridge was extremely tall for a pin bridge, and also angled massively taller towards the bass side.
This meant that performing a neck reset would only get me so far, as the bass side would remain extremely high regardless of neck angle. All things considered, the neck angle looked actually pretty good too. This meant shaving the bridge down to what would be considered a fairly normal thickness would get me 95% of the way there without risking major surgery and potentially causing other issues like fretboard chipping.
I'll upload some pictures tomorrow!
