Few questions..

[jcihos]

I have an M20 that does not seem to have any VIN stampings on it anywhere. Could this have been a factory replacement transmission?

Also, I have what was always told to me as a 1968 Z28. The original block is gone but from the parts that I do still have (heads, carb, intake, distributor, connecting rods, and pistons) they all seem to be specific to the 302. When I look for a block to rebuild the engine should I look for an MO stamped block (one currently on CL for $1k) or just go with a non MO stamped block?

[firstgenaddict]

What is the Assembly stamp formatted like this?    PXXXX

[bcmiller]

Is there anything stamped on the transmission? If so what are the numbers and letters?

Is there anything stamped on the pad of the engine block? It may have been decked. Check the casting number and date.

That sounds cheap for an MO block. Beware it may be a restamp. Post a pic if you can.

[jcihos]

This is what I found stamped on the transmission. P0404 date code. Also the main case number is 3885010, tail housing number is 3857584, and the side cover is 3884685. There is also a tag on the transmission with the numbers 3890534 on it?

The block that I was looking at is no longer on CL so I cannot post pictures of the stampings.

[ko-lek-tor]

Maybe you do not have Google, but it appears to be a M20 for a 65-67 Chevelle w/L78. It is amazing what is available if you would just put those numbers in a search.
https://books.google.com/books?id=CcK9tuyUC7YC&pg=PA43&lpg=PA43&dq=3890534+transmission&source=bl&ots=efoUFl1r-6&sig=gnihHfq5DXb7KexcCegre4vD9sg&hl=en&sa=X&ved=0CCsQ6AEwAmoVChMI8rONldS4yAIVgZeACh2x2gR6#v=onepage&q=3890534%20transmission&f=false

[jcihos]

thought it might be some sort or vin for the car my bad...