Over the history of RV Water Filter Store, we have had three different standard sized canisters as our primary offering. First was Pentair, then Kemflo, and now Hydronix. Unfortunately all of these have different o-rings! If you purchased your system after 2016, You probably have Hydronix canisters (our current brand) but they may also be Kemflo, since we were converting canisters during that time. If you can compare the pictures below to what you have, you see that the big difference is that Hydronix flares out at the top, and the Kemflo goes up evenly. If neither of those look right, the Pentair canister was the first one we used. These have opposite threads vs the other two, with the male threads on the sump and female on the head. This o-ring sits on the very top of the threads of the sump.
The story for the Jumbo canisters is simple...only one canister, therefore only one o-ring!
Here are the links to the o-rings
Kemflo Canister Hydronix Canister Pentair Canister Jumbo Canister