Nitrah;
As has been stated it is most likely a BA thread.
When I retired and decided to continue in my hobby of restoring English side x side shotguns I decided that I need to learn to engrave. Screws needed to be engraved and actions needed engraving sometimes. So I spent 3 years learning to engrave.
I enjoy helping others like you who have a problem, so if will send the old screw down to me I will make you one, engrave and black it. The screw slot may or may not exactly line up exactly as one needs the gun in hand to make the screw that way. If you will pay the postage both ways I will make it for you gratis. PM me.
Kindest Regards;
Stephen Howell
This offer from bushveld is incredibly kind and generous.
It stands in stark contrast to some of the gunsmiths here who do continual Free Advertising in their tagline, and then neglect to pay Dave for sales made as a result of those ads.
We have been treated to seeing some examples of bushveld's craftsmanship on several occasions, and his work is first rate in every way. It may not seem like much to some, but there is much more involved in making, engraving, and bluing a gun screw than one might think.
I am reminded of a story told by a gunsmith in one of the excellent Brownell's Gunsmith Kinks books. He described the process of measuring the screw size and thread pitch, choosing correct screw stock, turning the head and shank in his lathe to the correct length and diameters, setting up and doing the thread cutting, indexing and finishing the head, and finally cutting the screw slot in the correct position. He didn't do the extra steps of matching engraving or bluing. After finishing, he proudly showed his new screw to his wife, who had come into the shop. She looked at it and said, "Oh, how nice... you made a nail."
