You can pair them up by size, some of them are close but they are all different enough that with close inspection you can determine which are which.
We have been dancing around trying to get this right but I think there is no right to be found. Some folks like a nice tight fit and others like it looser. What we ended up settling on was putting the looser fit oring in the caps as they are shipped.
To compound things we did have to change orings on the caps as one order to the next on the caps has varied slightly. It used to be 9.5 x 1.5mm on the inside of both 19s and 25s then our new 25s needed #012 which were a perfect fit, the new 19s came in and needed a tighter oring because the 9.5 x 1.5mm were again too loose but many folks found the #012 too tight so we started using 10.1 x 1.6mm which is a very minor addition to the ID but also a little extra rubber to fill the gap and these fit very well. I think most will agree on this but then some still like the tighter #012.
Its easy to see here how close the tolerance is here and why they look very similar and how a very small change can throw things off. I have our caps spec'd at 0.002" +/- for the oring grooves. Any tighter tolerance and it would more than double the cost of the caps and I dont think anyone wants to see that least of all us because we would end up the proud owners of a lot of very precise caps
This is the cross section measurement for the three oring sizes we have used, these three sizes cover every cap we have sold for the inside to date including Acrylic caps.
0.0590" = 9.5 x 1.5mm
0.0630" = 10.1 x 1.6mm
0.0700" = #012 (#0** imperial orings are 1/16 inch 0.070 actual)
The problem we face is getting the correct orings out to folks who come back for a replacement, which do we ship? There will always be inconsistencies because of tolerance drift in both the caps and the orings themselves. I do know though that every cap has a cartomizer and refilling drip tip passed through it here before it ships. In fact the Mrs puts both caps on her test tube to check the outside fit then puts the carto all the way through then takes it out, takes it apart and assembles them onto the tank being shipped if they were purchased with a tube.
I think what we might need to do is put together a sizer pack with all the sizes and each pair labeled then everyone can choose the size they like for whichever caps they have to match the fit they prefer and know what size they need should they want to pick up new ones then also offer individual orings by size in a few different quantities. It just seems that fit is about as subjective as flavor and this leaves us at a loss as to what to provide hence the inclusion of multiple orings for the inside with every pair of caps.
I would love to hear more feedback on this because it really is easy to get a tank set up for any personal preference but I think we need an easy way for folks to be able to identify and what they like in the first place and label in such a way so it is easy to order more.