Mystery Movement... what do you think gents?

 That is an AS 340, it started in the first half of the 20's and versions were still made into the early 40's. It is a very common movement, and Bulova called it the 10B and 10 AL, although there may have been other variants. I've fixed quite a few of these. The AT came in two sizes, one was 10.5''' and the other was a pocket watch, 19.5''', called the 19AT. It looks like the 10AT is also a Bulova name for the AS340 series ebauches. 

 I did some more checking, the 10AL and 10B are indeed an AS 340, but the 10AT and 10AI are made by FHF, and are different movements.  I can confirm that the movement in your picture is an AS340. The FHF 32 (10AT) is listed as a 7.75''' movement and has a different train bridge shape, according to its original technical documents. This would suggest that it is not an AT, so that jives with what timerestoration has said. All my 340's look exactly like the picture, and I have dozens. The OP pic does look like it is stamped 10AI. In later years, ebauche makers under the ESA umbrella manufactured identical movements and some were indeed double stamped, (ie., FHF/ST, or AS/ST) but this would be very early for that. Pardon the deviation into minutia, but my own fascination is with the evolution of the movements and the companies that produced them. It is also possible that my documents are incorrect, and I need to know if there are errors in them. 

edit: I have found several AI's and they are 340's. Weird, because Bestfit lists them as being different.