Is this an oddity or just Mopar weirdness?

I recently pulled a 340 4bbl engine out of a 1970 Dart I purchased. Engine block has a casting date of 6-30-70 but motor build date is 336th day of 1971 and it is an HM designation (1972, Mound Rd.). Engine also has steel crank and 2.02 J heads although I have no way to know if they are original. Was it common for blocks to sit around that long before being built up?

Build Number is: HM340P 37800050