Why no numbers?

Awhile back I picked up 360 engine from a junkyard. It was in a 75-76 Dodge pickup. After I got it apart, I looked for the engine numbers. I looked on the front of the block just below the head no numbers. I checked the block just above the pan, no numbers. The casting date is 6-30-75. Does this mean it was a replacement block? Can anyone explain.
Thanks,
Mike