Educated guessing + process of elimination!
I also did a new thing in sanding some grooves out of the phenolic plastic caliper piston. That thing is probably OEM so it is 20 years old. I went online to see if there was any prior art to sanding grooves outta plastic pistons but found nothing. I decided that sanding had everything to gain and not much to lose, so that's what I did. Came out looking very nice and it seems to work well too.