Piston ID

I agree that they look like the old TRW 12.5:1 pistons. Comparing yours to Mr. Sinister's, though, it's pretty obvious yours had some custom work done. Other than the obvious mill marks, compare the plug pocket. Yours seems to travel further into the "valve" face of the dome, which is what I'd expect from material being removed there.

You could measure the depth of the valve pockets and compare them to what Sinister has to get a rough idea of how much was removed. Calculating actual compression? I don't know if I'd want to do the math involved finding the area of that area of the dome.