Bwaller- yes the pistons have a flat, no skirt bottom, and although the Ebay seller lists them as 60.5mm, I believe he incorrectly measured the top of the piston as I get 61mm at the skirt. I haven't looked at the rings, but my kit came with them...
Tin tin- also yes, this is the "605" kit from ebay for the 500. My understanding is that "ART" distributed "Henry Abe" stuff, or maybe the other way around, but my NOS box has got an "MC" sticker with a logo just like the old brit "MC" pistons. Even at 61mm though its not a 605 kit, or even the 600 casted into the pistons, my math gets me to 592.