Hi Calfed,
That looks like vertical stringing to me. (shooter error, probably not a gun problem)
I assume you shot that with the stock iron sights? If so, you have a great shooter. I can't get below two - three inches at 100 yards with iron sights, yet with a good scope and gun I can group below 0.5 inches. (1" on a bad day)
Based on your lack of side to side dispersion, that gun will shoot
2" groups as is with that ammo.
Put a 1" orange target dot in the center of that target and try again. You need to have a good aiming point and a firm bench rest to test your guns true accuracy. Make sure the gun is rock solid, watch your breathing, learn where the trigger breaks, etc....
How is the trigger pull? Mine took about 100-200 rounds for the trigger to wear in properly. It's sill a long pull, but not as heavy.
Shoot a few more groups before switching ammo. I handload for my Nagants with the Hornady .312 diameter bullets and they shoot very well. My barrels slugged out at 0.313" and 0.314" on the grooves.
Fantastic stock, and the gun looks to have the original browning (plum color) it probably is unissued, what a great find.
Have fun with it!
BWC