since you are probably ordering o-rings in bulk, I would like to suggest that in the future, instead of these black o-rings, you switch to red silicon o-rings from oringsusa.com. Their part number is: -008 S40
They sell them for 10c a piece (for a minimum order of 250 pieces) but for a large order they will do a better price. You have to call them up though, they don't show them in their online store.
These red o-rings are slightly softer (40A durometer) than your current o-rings and have the advantage of being easier to install (they can be rolled over the stem) and easier to locate if you drop them on a black keyboard.
I usually recommend the black ones to folks buying just 1 set, but if you are buying in bulk and don't mind the minimum order amount, the red ones are a bit better.
EDIT: I put more info and pics of the red o-rings, along with contact info for oringsusa.com here.