I guess the best way to get informed, is to contact Hagen directly:

Hagen Bøssemakerverksted Jan-Magne Hoel
Myrerskogveien 17
0495 Oslo

Telefon: 22 15 36 66

And how a Norwegian imported Merkel ended up in the US, is anyones guess. But one possible answer is that it was confiscated by the German occupation army (happened to all weapons in Norway) and brought to Germany during the war. Later it was probably taken to the US by a US soldier.