WoW !! That is weird, Wilbur. I'd have to go back and see where I got mine from, give me a minute and I'll find out. Oh, I have bought (4) of them and all had that o-ring on them for both size tanks, I think from EightVape and GearBest, and 3fvape also. So I got one on the 2ml size and one on the 4ml size, plus an extra in the spares pack.
Edit; did you check in that black foam also ?? Ya know, it could be just different assemblers putting them together, and some of them put on that extra o-ring, and some others figure that you can use the one o-ring for both the different tank sizes, and swap it back and forth, plus the one extra in the spares pack. (I got (3) altogether of the 5x1mm o-rings per box). I bought (1) Black and (1) Gold for me, and (1) Black and (1) SS for my daughter.
Again, don't forget to lube up that o-ring (and ALL your o-rings for that matter) before you screw in the Chimney to the top cap. I always use just a dab of plain VG for lubing o-rings cause it's thicker than PG and will stick onto the o-ring better and longer, but you could just use a dab of your e-juice and that should work out just fine.