I've seen a lot of sets listed for sale where the seller says that the hoops are COB die casts but these are usually Gretsch hoops which are known to be zinc. I think some people simply confuse the die casts with the stick choppers, which were made of brass. Even CW used to describe the hoops on some of her Gretsch sets as being "COB" when clearly they were standard Gretsch cast hoops.

"Brass" and "COB" seem to have aquired the same status as "maple" in the vintage drum lexicon. Nothing wrong with that, I prefer COB hoops myself (but those aren't cast) but people seem to try to stretch the use of the term(s) when describing a drum they have for sale.