Scally, thanks for sharing these unknowns. Some interesting cartridges. They will take a bit of analysis.
This one is definetly an ‘MB’ type but doesn’t exactly match any of the 152 variations that I am aware of. I agree with Fede that it is a cut down 11.5x50R Werder. It has close to the same shoulder position as the Long neck type (MB100 in European Sporting cartridges) which was produced by Egestorff and listed as No 11550 MBa (nach Utendoerffer) rather than the No 11550 Werder (nach Stahl) that you mention Fede. Note that Egestorff listed three types of 11.5x50R ‘Werder’, the other being the original 11.5x50R B type which is the very original M69 Bavarian Werder with an even longer neck as originally produced by Utendoerffer.
The closest MB case is the 11.3x48R Austrian (MB140) which in effect is also a shortened version of the Werder. Originally produced in Austria by G.Roth as case #211. It has the same shoulder position but has about a 2mm shorter case than the 11.5x50 and a 1mm longer case than this Egestorff version.
A similar c48mm Utendoerffer case is known which is known but this was never listed by Utendoerffer (or any German company for that matter).
Both the Egestorff and Utendoerffer cases may well be attempts to produce MB140 and that is my best explanation at the moment -It is likely to be a private attempt (ie non-factory) at producing the 11.3x48R Austrian MB case.