I think that's the US guy's new web page, hopefully a direct link to the R65 piston. Does that measurement look right? Arvo, it looks like you have some handy to measure.
Thanks for the link. You say these are stainless? I couldn't find any mention of material on that page.
I can't find anything about the material, either, now. I don't remember why I thought they were stainless. Email the guy and ask him.