Inliners International Forums General Discussion Engines engine suffix code

I've got a known- to- be -original '68 292 block, stamped numbers are FO627XAE. This engine was in the truck when I bought it in 1973.
I've wondered about the "X" myself.LMC Truck has some partial codes (and vin codes) broken down in their catalog. They don't list any X. I wonder if it is a dealer- exchanged engine under original owner's warranty? Just a wild guess.
My other block is stamped FO626UH. LMC's chart does correctly ID it as a '67.