I was also in search of the elusive 11.15x65r for my Sauer hammer drilling and had contacted Steve Meyer of the German Gun Collector Assn. I have included his reply below. I am also posting a picture of the bottom of my barrel that I shared with Steve as well.
Hope the following information helps you.
The 11,15x65R (also noted as 11,15x65,4R) cartridge is an elongated version of the German military 11,15x60R (.43) Mauser round. The .43 Mauser spawned dozens of sporting cartridges of various calibers and lengths and the 65mm version was fairly popular, but it really has no relation to any .450 round. The 62/78 indicates the gauge of the rifle barrel and is usually all that's seen to give any clue as to the chambering - having the explicit "patr(one)" (cartridge) mark is rare. Still, a chamber cast wouldn't hurt. 62.78 gauge translates into .420", which would be the largest diameter rod, in .010" increments, that would fit in the bore. Groove diameter of your rifle barrel will probably be somewhere between .440-.448"
The rest of that cartridge mark shows that it was loaded with black powder (SP = schwartze pulver) and the bullet weight was 17,7 grams (about 273 grains) and plain lead (BL = Blei, or lead).
Unfortunately I don't know of a source for formed 11,15x65R cases. Huntington's lists .44 Sharps basic brass which could be used as a starting point, and I'm pretty certain that judicious use of .43 Mauser dies would result in usable ammo without the expense of having dies custom made. Buffalo Arms is also a good source for unusual brass and advice on BP loading. They don't list the 65mm case, but they do 'stretch' brass and may be able to make some for you.
