Lol ok here is what I ment to say Sallyem sorry about that.
In my experience there are 4 types of condom/butplugs
one with no nub at all or stem in the middle.
one with a small nub
one with a large nub
one with a stem.
I use the one with no nubs or stems to refill, and I cap my refilled cartos with the nubbed ends, I never use the stems at all anymore.