Husqvarna had their own proofing, no Swedish proof house. There is a listing of the marks that use to be available on line, and maybe still is somewhere. The 20B is the model, the 12 is the gauge, the K on the barrel flats would be the mark of the finisher Claes Johansson. If it is an X it was Erik Isaksson.

If the gun was really from 1942 in may have 65 mm chambers. After WW-II they went to 70 mm chambers.