No one has said this yet, so I will.

When Mamiya was actively selling RB stuff, it had a FAQ section on web page. It basically said what the ring was there for but it also said it isn't necessary for a proper operation of the lens that no light leak will result for not using it. I used to think, then, why the heck the ring costs over 30 dollars??

You might just want to call KEH and see if they got one for less...

By the way, the ring isn't like a gasket. It does not make a tight seal or anything.....