I had much the same experence with my O/U in 16ga./9.3.
When I bought the gun it came with 8 rounds of DWM ammo and the sellor told me it kicked too much for him to shoot.
When I shot it, the recoil was rough but then the gun refused to open and when I finally susceeded the primer was blown and extruded back into the firing pin hole.
When I slugged the barrel (something I should have done before firing) I found the groove to be .358. I have not been able to find any reference to a 9x72R but the good thing is that there is a large selection of .358 bullets and the gun shoots cloverleafs at 100yds with 200gr. Rem,SRA, or HDY over 38/IMR 3031.
The fact that the gun fired a .366 bullet down a .358 bore without any apparent damage is a testament to it's construction.
That was over 40 years ago and it has been shot countless times since then and you still can't see the line where the barrels/frame come together.
No idea on the maker as it has an added scope rail inset into the rib where the maker's name would have been.
Dave