Just a general knowledge question to the community.

'68 400 motor, rebuilt about 3,000 miles ago by owner #2. I am owner #3.

D298 code on block (April 29, 1968 production date)
D298 code on right head (same day of production)
D048 code on left head (April 4, 1968 production date)

I had read that the head codes may not match, yet still be original for that block.
Did they have a pile of age appropriate heads to grab from while building a motor at the factory in 1968, or is this April 4th 1968 head a replacement over the car's 50+ years ???