I think they are called Helikons or something. I don't think the horns are just decorative. I picked up one like it this weekend at the accordion festival, and the horns on the bass side certainly gave out a very verbose OOM PAH PAH.
Daar mis je niet veel aan Dwight!
In other words: You don't miss much, Dwight.
But here's what I said:
I found an accordion with "trumpets" on it in a 1913 catalogue of Leonidas (a kind of Dutch Sears Roebuck).
translated:
"finest concert-harmonica with big copper horns, which give a beautiful heavy trumpet sound".