Yup, it's square with a corner cut off. A bunch of research is available online. I'm aware of all the obvious topics and was not wasting anyone's time. I have new questions. I'm pretty sure the pan is from a '77 TH350.

UPDATE: i found partial vin on Driver's side just above pan. It's a match to the car's vin. It is a '69 TH350 original gloss black (dirty) and works super nice. Shifts a bit quick into second and won't stay in kickdown for more than a couple seconds.