I'm pretty sure the .308 or 7.62x51 (reloader will argue they are not the same) came about in early 1950's. If you, indeed, have an original 1916 Spanish Mauser it should be chambered in 7x57mm (or "7mm Mauser"). Some were subsequently rechambered to 7.92x57 (or "8mm Mauser"). This should be denoted on the rifle somewhere, like the receiver. My source of info. is Ball's "Military Mausers of the World."
"Wars are, of course, as a rule to be avoided; but they are far better than certain kinds of peace." - T.R.