Stop in for a cup of coffee

Ok so here’s a riddle for you to solve.

Stopped at a gun shop yesterday in Kentucky on the way home. They had an 1898 Gewher Mauser. Original rifle but had been rechambered in something that they’ve not been able to figure out. Has a 7.62mm throat. However, 8mm Mauser, 308, 30-06, 7.62x54R, 7.62x39, 7mm Mauser , none of those fit or even chamber.
Could it possibly 7.62x39 ? I had a friend that converted many mausers to that several years ago.