Ok, thanks Mark.

Coming back to this. I do appreciate the responses. I do not see a "K" on the top of the piston. It is indeed a Yamaha piston as I see both a raised "Yamaha" and "Izumi" underneath on the skirt.

When I ordered new rings, I actually ordered both regular and keystone ring sets (hedging my bet, so to speak). Neither would fit in the piston ring lands.

For the moment, as i really need to make some progress on this one (been on the bench too long), I reinstalled the old piston and rings and the cylinder slipped right back in place. I installed the magneto DEET redid for me, and as it has only the one spark plug hole in the head, I do not at this time have the head in place. Going to time it up tomorrow, but after that is done, I may (or may not, depending on how lazy I am) remove  the cylinder again and try testing the rings as you suggest.

At this point I see this as more a puzzle than a problem. But sometimes puzzles turn out to actually be problems later on, so maybe I need to do this. 

Thanks for the info guys! 

With keystone piston & rings were both rings keystone or just the top one. I'll go find some to check later. Does seem odd that neither set fitted. Did you remove the expanders behind the old rings. They get well carboned in with old age & miles with vintage types of oil.

sounds like you got a nice fresh bore and a clean piston, so rings are probably as good as new!
I think you are in good shape 

Part number for plain rings use a 10 suffix for first oversize .
I'm unsure exactly what your 12 suffix actually signifies , but
13 suffix is quite common , and may be what you need .

My DT1 is out to 4th over . I used K marked piston with 43 suffix rings .