Got a chance to mess with it again tonight. Sure enough there was on old ring in it. Had been so hardened and worn that it had formed to the groove.
The nut on the bottom of the cap spins just fine, but right before it would grab on the new ring it would turn a 1/4 turn.
I know many of you will cringe, but I went with the advice that said to leave the old one in. Isn't leaking and I can get back to work.
As for the size of rings, it did come with 2 I threw out the larger one right away.
I should go to my cabinet in the shop and dig out the manual for it, but what is the rec. interval for oil changes. I've been going on 200 hours (estimate, obviously no hour meter).