Cross-posted to Britishmilitaria forum as well -
Bertram of Australia makes 10,75x65R brass which is available from Huntington's, MidwayUSA and probably others, and although they don't explicitly say it is the Grundig version I would be surprised if it weren't.
Per Dixon:
case length 64.77mm
rim diam 13.7mm
base diam 12.48mm
neck diam 11.48mm
commercially loaded from 1900 to 1933 or so by Utendoerffer and Gecado
Dixon's comments:
"This caliber was listed by RWS/Utendoerffer and appears in their 1908-1921 catalogs. Also listed by Gecado into the 1930s. This caliber is not to be confused with the 10,75x65R Collath which uses a thinner case but like that caliber appears to have a very slight neck."
