You should slug the chamber with cerrosafe or similar product, to confirm the caliber. It is most likely chambered in .43 spanish, but could be .43 reformado, .43 egyptian .... .43 spanish, was probably the most common military caliber. Except for rifles, from the middle east, I haven't seen military markings on most rifles. (I am in no way, an expert.) What condition is the bore? They are fun to shoot.
John