You can buy a 5.5mm x 0.5 die from China for about $11 postage paid on eBay. Whatever people say about Chinese thread tooling, it will be amply good for one-time use on a tang screw. 5.5mm is about .2165in., and the external diameter of a Unified No.12 screw is .216in. I think the Brownells blank screw kit in No.12 would be long enough, at $6.95. If it isn't, the new screw would have to be turned. But the Brownells ones have a very narrow slot, usable as-is, or which you can widen. I use a fine hacksaw blade with the ondulations of the edge belt-sanded flat. General-purpose hardware screws have a wide slot on the principle that a screwdriver needs to outwear hundreds of screws, but a narrower slot and blade suits gun work better.
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It has to be acknowledged that Miroku do some of the best factory gunsmithing, in measurements that aren't exotic to them. Muzzle not the ox that treadeth out the corn.