"trial" single shot Mosin(1890)
"regular" Mosin(1891 - so called "Mosin-Nagant")
Single shot(top) and "regular"(bottom) mosin bolts:
and nagant bolt:
Before mosin, russians used Berdan № 2:
Yours "mystery rifle" does not look like any of them, so nothin russian, but bears some similarity to other bolt action single shot rifles/carbines of "Berdan № 2 era"(that is 70's of the XIX century - early 80's if you count ones fitted with a tubular magazine).
It is not M1871 Beaumont, M1884 Jarmann, Murata Type 13(real "frankengun", Mauser/Beaumont -probably Gras too - "mix"), M1882 Chaffee-Reese, Mauser mod.71 etc.
There is slight(really minor) resemblance(mainly in bolt construction) to Hotchkiss rifle and Gras Mle1874:
And what if it stared its life "as is"?
Especially it seems to be quite small gun (most mentioned above use ammo in "45-70 class").
I will quote kuduae from nitroexpress.com (it is about sport/schutzen/rook rifles being simplified and often miniaturized m71 mausers - I don't think it is a case, but principle is probably same)
the only 8.15X46R's were on ADYT or AYDT (?) single shot rifles.
No, Daryl. After Adolph Frohn, Suhl, designed this cartridge after 1890, it soon became the standard German centerfire target cartridge and remained so until WW2. It's main advantages were: It was available everywhere. It was easily reloadable, pre-packed powder charges and swaged lead bullets in many weights available from the ammunition companies.
So it was chambered in all types of rifles, even some combination guns and drillings are known. Tom Rowe published 2 books, "Alte Scheibenwaffen I and II", on the various German target rifle actions. Though most of them were Falling blocks like the Aydt, the various Büchels, Nimrod and so on, others were variations of the basic Martini action like the Frohn, Stahl or Kessler. Nearly all these actions were made in 8.15x46R too. Cheaper sporting rifles, used for less formal target shooting and small game hunting up to roe deer, in 8.15x46R were buit on Tell-type break-open actions and these simplified Mauser M71 type actions. If you look at the catalog pages shown by Lancaster in this thread, you find the Normalhülse = standard case 8.15x46R among most caliber offerings.
These simplified M71 type actions are not sporterized Military actions and were not made by Mauser. Instead, they were a staple of the Suhl and, even more so, the Zella-Mehlis guntrade up to WW1. Usually they omitted the M71 bolt stop screw, the sear serving as a bolt stop, and often the Mauser flag safety too. They were made by many makers there in all sizes from giant for 12 gauge Vogelbüchsen to tiny ones for pistol-sized rook and rabbit cartridges. Here is such a miniature action by Sauer & Sohn, pre-1893, in 7x20R "Z", essentially a .32 long Colt case tapered down. As you see, the action is much too small for the 8.15x46R, top photo, but would fit a .32-20, bottom.