When these were introduced by Chrysler it was in all the Mopar magazines. The story went, that as Chrysler stopped production of bigblocks in 1978 all the bigblock stuff was scrapped. So when they decided to make a new Hemi block they had to start from scratch. This was the result. The "M" after the casting number was to keep them from being passed off as original blocks.

I am guessing that these were the blocks that MP also used for the new crate Hemis in the Cummins era, but I've no proof.

I do know that there was much rejoicing in the MOPAR world when this announcement came out. Plus the availability of Siamese blocks opened up a whole new range of displacements available.

R.