i ordered mine off a google search of the number on the original o-ring, the diameter should stay the same for a 20" rim, the only real variation is the thickness or round vs triangle, 4 outta 6 of my steelies were new and 5 had triangle rings, one has a round ring, my new rings are round that i got when one decided to leak, i'll check my phone and see if i can find a trace of the number i ended up with, if i do i'll post it right back, if not i'll post the info from the package of the spair ring at the house after work.
edit this is what i got, don't remember the original number but it was short and simple, when i tried local tire stores they wanted to sell me 22 or 24" rings and to cut and super glue the ends together
edit edit, when i look closely that o-ring you have looks like the right size, it doesn't sit on top of the flange, it goes around the barrel of the rim and sits on that step below the bead, i greased up mine with a thin coat of silicone grease