Response to UTG and the comment section on Piston Knurling ‪@UncleTonysGarage‬

Yes, knurling (both pistons and valve guides) was kind of the norm "back in the day". Not so much as a performance mod, just a regular service ritual on higher mileage engines to eke out some more miles out of an aging, but otherwise good condition motor without a lot of taper. Remember, these things were VERY lucky to hit 100K miles back then- not the 2-300K engines now. VERY common on marine/industrial/AG engines.
For its intended purpose, it worked fine. The issue today is finding a shop that even does it.