it's the one
the HDS tubes take kind of an oddball o-ring size. i managed to rig one using a verrrry stretched ring of another size, but it just puts too much strain on the ring to be long-term
this, on the other hand, is the right size (big) and thickness (tiny) for the tube - plus it's a nice, firm rubber (or nitrile?). 50 cents is a bit much for one o-ring... but it's the one.
Reviewed by: Emilio González from Silicon Valley.