Without measuring it, I can't give you advice on the way to go. It's certainly a pre SAE, ANSI, or MIL-105 thread size. I'm guessing it was a 3/16"-36 (if it is 36 tpi). Pitch dia might have been something that was sorta standardized among certain industries or within a country or region. Maybe even a British thread.

If you have a friend with a machine shop, he may have an optical comparator. This is basically a projected light with a focused, magnified shadow image displayed on a frosted screen. The screen has measuring increments and angle measuring capability. This will identify the thread flank angles and pitch redily and can even measure pitch diameter if it's a good one.

Failing the availability of an optical comparator, a set of pitch gauges and some pitch wires and a micrometer and you'll be able to learn enough.

If you find the pitch to be 36 tpi and the pitch diameter to be slightly under a #10-36, Rutland Tool has a 10-36 split button die for $12. http://www.shoprutlandtool.com/webapp/wc...1=&scFlag=0

Be darn sure it's a 36 before you run the die on it or it'll make a mess. And remember, a button die has a cutting lead that won't cut full threads to the shoulder of the screw unless you flip it over and use the 'backside'.

To hold onto a screwhead that has a straight o.d. like most action screws, drill/ream a nice hole the same diameter as the head of the screw in a small block of aluminum or brass and split the block on one side with a saw. Put the block in a vise and tighten with the screw in place and the threads protruding upward.