What are these marking?

I know what the markings are. Those are the engine bore specs carried out to the fifth decimal place. They are only published in the FSM for the 426 Hemi. Example- 69 FSM, page 9-98 Cylinder Bore (standard) "A" Size 4.24975" "B" Size 4.25025, etc. There must have been more information at the engine plants for all the engines.

I found the same thing when I re-ringed my 383. A testament to Chryslers engineering and machinist history. They must have had a piston-sorting system that was very precise. They measured the bores out to four or five decimal places, and the pistons as well. A slightly larger piston was matched to a slightly larger bore. We're talking about a range of .002. the 426 Hemi section lists 5 sizes, "A" through "E".