The headstamp is T C N or some combination of those 3 letters. Is this Turkish?
The case has all the dimensions of a normal 11.15x60mmR Mauser with the exception of the small primer. The long lead bullet has a coating of what looks to be dried grease or lubricant of some kind which actually has a brownish tinge but it looks grey in the scan. The cartridge is heavier (691.4 grains) than a normal German 11mm Mauser and has a .441 bullet.
Anyone know where it came from and/or who made it?