I think it safe to say that the "proof" marks have no more validity than the other markings. The rifle sounds like a Khyber pass special and my recommendation is that it be considered a wall hanger. The .303 British (and the .30-30) are somewhat objects of scorn by those who like "hot" cartridges with higher pressures, but neither is anything to toy with and both are fully capable of suddenly disassembling a rifle made from unknown material by an unknown person in an unknown location at an unknown time.
The threads, I suspect, are neither English nor Metric. They are "what looked right" when the craftsman made his receiver tap and set up his foot powered lathe.
I may sound like I have no respect for those guns, but in fact I have a great deal of respect and admiration for people who are so determined to remain free that they can and do manufacture their own weapons, and even if they look odd and have funny markings, they shoot, often very well.