Ring pulls

Midwestern Brotherhood of Steel uses ring pulls as currency when trading with outsiders (non-brotherhood) in Fallout Tactics. Ring pulls can be found all over the settlements in early missions and traders will trade for them. There are usually some outsider traders in Brotherhood bases who will also accept ring pulls.

Ring pulls are likely used as currency for the same reason that bottle caps were in the first Fallout game, being that they're harder to reproduce with post-war technology, thus reducing the odds of counter fitting.

Appearances in games
Ring pulls only appear in Fallout Tactics