It does look like an old Mega Block, does have the dash M casting number, and is clearly finish machined as a Hemi block with pushrod clearance, inner row head studs, and original style engine mount pads. So, the only thing we don't know is if it's a street block with water passages between the cylinders or is it a siamese bore version. If heads or core plugs are still off/out, just look through the coolant passages to see if there is a passage between adjacent cylinders for coolant. There were several part numbers available, but they varied based on siamese or non-siamese, original style or 440 ears for engine mounts, and which bore diameter it was finished with.