Well, it's a Megablock.
Beyond that, there isn't much to ID it because not much was required in the way of serialization. It just wasn't important way back then, I guess. Here's your block, have fun.
Mopar's normal past practice was, take a casting and machine it as many different ways as possible (to get as many different part numbers as possible from one tool).....however once the box is gone there's no telling what part number it was originally sold as, so there's where the "have fun" part comes in. Ex: your casting number could have either been a Wedge or a Hemi depending on how it was machined.
Unfortunately things are never quite as simple as "who made it" because of the multi tiered supply chain. MC is the foundry mark. Far as I'm aware MC did not do machine work or packaging, they just sent raw castings along to whatever the next step of the supply chain was at the time.
The modern blocks have many more identifying marks, and are serialized in such a way that normal human beings can read it and make sense out of it without a secret squirrel decoder ring. And if that isn't enough, the packaged part number is even engraved on the newer blocks, so no need to save boxes for 15 years anymore to remember what part number you have
Progress!
