I hate to write one of those "what XXX is this", but I've been looking for a couple days now and I can't seem to figure out what I have. Maybe I've just been confusing myself. Anyway...
It's out of a 69 firebird with a 350, 2bbl. It's a 2 speed.
From what I've read, most 2 speed firebirds had the ST-300 transmission. But, the pan on those is supposed to be square at the front, and taper a bit towards the back. This has a square pan with a corner chopped off.
I tried to make a picture that shows the markings that I found, and where they are.
I'm guessing this is probably a replacement transmission from a while ago...but I don't know what kind.
The real reason for asking is to get a pan gasket, filter, and front and rear seals for it. Easier to do the service while it's out.