I think Jansen was more a retailer and would bet this double actually was made in Liege ...
like many other british rifles.

would be the same like most of the german court gunmaker who get their guns from Suhl. Jannsen was the leading outfitter in belgium for anyone going to africa.
this here is a 12,7x70(500 jeffrys) sold by Jansen once into the belgish congo and than for decades in the hands of Adrian de Schryver there.
http://www.youtube.com/watch?v=MKkZawWUqTQ