I'll agree with Raimey as it stands now with the picture of the chamber cast at http://britishmilitariaforums.yuku.com/topic/7111

Dixon mentions that both the Collath round and the Grundig do show small shoulders, with the base and rim of the Grundig being closer to the cast.

The fly in this ointment is the 1943 date estimation. The Collath rounds, according to Dixon again, were not commercially loaded past 1915, and production of the Grundig round ceased in the early '30s. I can't find my reference to Belgian inspector codes (the star-over-V) to cross reference the date.

The 10,75x68R Mauser seems a bit long for the pictured cast which shows a 2.603" (66mm) length to the end of the chamber.