AMC242 pistons have "D"-shaped dishes in the deck, probably to reduce overall compression ratio with the high displacement. They are also quench complaint (meaning that slightly higher compression is possible with low-octane fuel than normally doable.)
You should note regular and consistent "D"-shaped depressions in all six piston tops. The use of truly "flat" piston crowns in automotive engines is increasingly rare - I think the last time I saw a flat pison it was in a powerplant for a LeRoi industrial air compressor.
5-90