I have a set of Ross (#425) like-new pistons that came in a 427 I recently bought. They have a ridge (step) about 1/8" down from the top. The top of the piston measures 4.18. Below the step the measurement is 4.20.
I don't recall seeing this on any other 390 piston I've ever bought, but maybe I just don't remember well. This is my first 427 if that's relevant.
See the little 'arc' above the valve reliefs? That's on every piston in the set. I think it was assembled incorrectly and the engine was never run after initial startup.