A friend is describing this to me so excuse the unanswered parts that I know should be included.
A Belgium rifle dated 1943 (with the v and a star over it) with 10.3 stamped on the barrel, Nitro proofs post 1924.With a rim daimeter of .550 and a base of .500. The bore is .420. He "thinks" he can feel a end of chamber length of 2". And it is a straight walled cartridge.

I've looked thru COTW and Cart. Conv. by Donnelly with out luck.
Any ideas???
Thank You Gentlemen.