Get a gunsmith to do a chamber cast and slug the bore, then you can measure the casting and identify the chambering. The smith can possibly also help you identify the rifle.
Not to start an argument, but it sounds like a parts gun made with a pre-WW2 civilian 8mm J-bore barrel, which could be chambered for any one of several civilian 8mm cartridges (8X47, 8X51, 8X53, 8X57J, etc). If it was built new in the 1960s, it would likely not have the .318" bore, as that bore dimension was pretty much dead after WW1, and definitely obsolete after WW2. The bore dimension coupled with the barrel markings point to a prewar rifle. Some bullets are available (Woodleigh), but it is an odd size.