They are called gremlin bells. I actually have one. A very good friend of mine gave it to me... It is about 3/8" in diameter, and about the same height.
I look at it from the JUJU perspective. If it adds a bit of "luck" then why not... It's not big enough to be obtrusive... And aren't we all brothers to some degree? Kinda like Roadies VS mountain bike guys... :hammer:
www.gremlinbells.com