When Hemis were very plentiful in my part of the world and a hemi went to a rebuilder, the rebuilder would sometimes put a serial number on a motor to mark the parts. You have to remember a lot of hemi motors do not have serial numbers on them. The marking of a serial number on the Pad on the front of the block is easy to recognize for the assembler. I know I have marked the rods, crank, balancer and other parts so when I get it back I know the motor has the parts in it that I gave them and not someone elses parts. The number on the pad is it the number on the VIN Pad.