https://goo.gl/photos/oY3ErRZCMbNQx8X39
Ok. Bought it. Yes, the third speaker is indeed a G12M, had the T1221 code on it, with 1777 stamped on the cones. The cab sounds GREAT, particularly with my Strat through a Sig X. They are 75hz G12M 25W speakers. The cab dates back to 1977. The seller bought it from the original owner who ran a studio in NYC... he has seen the original receipt for the cab, which was bought new with a 50W head.
The badge is missing, obviously.
Here's the fun part, though. The guy who sold it to me just moved from NYC a short while ago. He's worked in studios for the past several years, most recently owned a studio out there. According to the him, this was a house cab at a major studio in NYC, and has been used for tracking by Vernon Reid, John Petrucci, and some others in the mid-late 1980's. There's really no way for me to verify if this was really used by the people he name-dropped, but it certainly seems possible. The name of the studio escapes me at this point, but I can contact him to ask him again.
The blue speaker was put in in the early 2000's after that Blackback blew out. He's not sure what it is, but it was put in by the house tech at the studio. It too sounds good. I've put a picture of that speaker in the photo album above, will spend some time tomorrow to try and research it.
I think this cab is a keeper.