That is a Mega block, not a OEM Mopar hemi cast block
the cast in M being the key
I look at that picture a little closer and maybe that is NOT a M
I have a Mega Hemi block motor in my shop now in a 1969 R.R. so I will look at it a little closer later, it definitely has the cast in M on the passenger side, I'll have to look at the casting number on that side of the block