Looking through Dixon's book on European cartridges, there isn't a lot that looks close.  The thick rim indicates a Mauser "A" base style but it is a smaller diameter, much like the Tesching target cartridges.  Most of those, however, have a base diameter of 13 mm, instead of your 11.6.  The closest one is the 10.5 x 40R that was only made by Roth from 1878 to 1914.  The timing is probably right, but certainly not sure on the exact case.  It will be something that you have to play with.  Good Luck.
Bruce

I can see two ways to go with your chamber but, either way, you'll need to get some 303 Brit cases and shorten them to a minimun of 1.77 and expand the neck so that they will chamber with a little resistance. Then fire form them with 10.0 gr of Unique, topped off with CoW or corn meal. Hold it in with a grease wad.

Inspect them to see if the mouth of the case is beveled. If it is, shorten until that gone. Since your chamber cast shows a sharp transistion from the neck to the throat, at the 1.77 it should just show that transition point after the case fire forms. If not make the next one a little longer.

If you do the case that way, it will head space on the rim, like a 30 Carbine or ACP pistol case.

If you want to head space on the rim, it vwill be a little expensive but, get your 303 case, measure the rim thickness and go nto a machine shop (or friend with a lathe) and have them turn and bore spacers to give you a total rim thickness of .097.

Frank

I have one similar
Mine is a 11.5x35 werder.  Using 43 Mauser brass and using .452 cowboy bullets.  16 grains of 5744 overall length of 2.006