Not dumb at all, in fact, a very frequent question.
I wish there was a good answer.
There are commonly 5 different size/supplier caps available:
CCI #10, CCI #11, Remington #10, Remington #11 and RWS #1075 (which the manufacturer claims is a #11).
Unfortuately, in my experience, those numbers don't mean a whole lot in terms of relative size and/or suitability for a given gun or nipple cone. I've measured several of each and found no relationship between the numbers and the brands. Here are the results (the first number is the inside diameter at the opening, the second number is the overall height, both in inches):
CCI #10: 0.160 0.160
CCI #11: 0.166 0.162
Rem #10: 0.167 0.168
Rem #11: 0.166 0.145
RWS #1075: 0.165 0.154
I should mention that the inside diameter measurement is very difficult to make reliably, so the precision in that number is very much less than the three digits implies.
I would assume from your results that you were using CCI #10's. If so, virtually any of the other four should fit fine.