This might not be unusual at all, but I discovered a plate block (airmail Scott C88) in a lot that I purchased, that has two completely different plate numbers on the same plate block. The numbers are right next to each other in the margin underneath two adjoining stamps of the block. I have not been collecting stamps that long, but this just seemed unusual to me. What is the reason for this?
Could you possibly mean C87, the 18c Statue of Liberty airmail and not the 26c Mt. Rushmore stamp? Otherwise, an image of the plate block would be appreciated.
If so, that is normal for the 18c. This was the beginning of the time when each color got a separate plate number alongside a different margin stamp. The 26c was released a few days before the 18c and may have been printed before this new plate number system was started.