Hellhammer was from Switzerland and is considered one of the pioneers of the genre of thrash/black/death metal even though these genres are generally associated with the nordic countries, so it all kind of get's mixed together I guess. They takes themselves far too seriously to almost comical lengths. Like, I was watching a documentary about this style of music and it was like the main dude from Hellhammer, now Celtic Frost, was trying to turn every memory into a saga during the interview.
I would have guessed it was a crazy Japanese band, that's were you are isn't it Rockers? Are you doing their merch because they are going to tour Japan or just from reputation?
As to the original question, I am pretty sure that drinking horns have never been mass produced.