[rechinca]

Check for the original paint on the block, but best bet is an older (probably 80’s vintage?) GM Targetmaster replacement block/engine. That is a generic casting number that will only you basic idea of engine. It was used for early 327’s and 350’s starting in 69(?) on up to the late 70’s in passenger cars. It also the reason the head pad has a weird stamped number. Normally they would read as a series of letters and number like listed in the nastyz28 site. To the left of the number would be partial VIN for the vehicle it was installed in if a factory installed engine. The alpha-numerical sequence where yours starts with an “m” would tell you the engine foundry/assembly plant, what plant it was installed into a vehicle at. That most definitely is not an original engine from the factory. Probably a crate or GM dealer, non-warranty replacement.

Side note though, if you find and engine you have an issue decoding, email the casting and head pad stamping numbers to the GM Heritage Center. It will take a few weeks, but they will email you back to confirm year/make/model, engine specs and where assembled/installed. I have done it for a couple of engines now and they are more than happy to help you out.