There were two different design pistons used in the 1968 302. The first design was a carryover from 1967 and had a part #3927177. Sometime in production they started using a second design piston and it had the part #3946876. My 1968 07C Z28 had the second design piston. I have never seen a part number stamped on the piston. There was a very marked difference in the design of the piston top on the two pistons. The early piston looked identical to the earlier 327 HP piston. The later piston looked like the 1969 design....BTW the late 1968 and 1969 pistons had the same part number.
One other detail....everything I have seen written about the second design piston states that it is a floating pin piston. But...when I tore down my original 302 block it did indeed have the late floating pin piston but the pins were pressed into the rods. Don't know for sure, but maybe they ran out of the "Pink" floating rods and had to use some "Pink" pressed pin rods when my engine was assembled. It was assemble on June 19, 1968.