Hi Steinar,
Some additional info from my old catalogs:
Remo was the factory mark for Gebr. Rempt Jagdwaffenfabrik in Suhl, established 1865. They made all sorts of hunting rifles including a .22lr version, a 3-5 shot version for different types of rimless cartridges and a 2-shot size 12, 16 or 20 version, amongst others. The company was alive and well in 1937 and it also seems to have survived the war, being mentioned in a 1952 catalog. Although the author of the article above has doubts about size 20 shotguns, the catalogs mention they were available.
The conversion of the K98 to a shotgun was patented by the Rempt company in Germany in 1919. It was patented under #328446.