Two questions. Are you talking about an actual sierra cup (tapered sides, rolled edge, wire handle), or any generic handled metal cup that might be called a sierra cup? And is he sure it was brass, maybe it brass plated or possibly copper?

I was thinking it could be possible that it's an older style of soldiers cup used by one of the military forces or a reproduction of some sort. Similar to one of these.

http://www.fcsutler.com/fctin.asp

http://goosebay-workshops.com/CUPS-CANTEENS

http://www.alberene.com/prodinfo.asp?number=PBN4700

I just did a search for brass soldier's cup, or brass cooking cup. If you're willing to spend the money, you can always have them custom made. If you have a picture of the one you're talking about, I'll try and keep my eye out.