Maybe this is common knowledge to everyone else but it news to me. I bought a Turnbull 1886 Winchester copy a while back. More recently I decided to put a tang sight on it. My eyes are old and the tang sight helps greatly. In mounting the tang sight I found the rear tang screw was not long enough. My thread checker seemed to indicate that the screw was a 1/4X40 thread. I can make a screw on my lath but I had difficulty in getting a 1/4X40 die so I emailed Turnbull to verify the thread and also ask if they had screws for tang sights.
What I found out is that the rear tang screw on the Turnbull 1886 is metric thread. (5.5X05) When I asked, "Why in the world would someone use an obscure metric thread on a Winchester copy?", the answer I received was "The base for our Turnbull 1886 rifle is the receiver made for Winchester by Miroku in Japan. We get them in here and the first thing we do is complete the tang safety conversion and half cock conversion like an original 1886. We also drill and tap the second hole in the tang for the tang sight and tap it to original 10-36. We cannot change any of the metric screws."
To my way of thinking, their first mistake was using a Jap made receiver for another company (Winchester). Second mistake was Winchester using metric threads for their own copy. Had I known this prior to purchasing the gun I would have gone another route, for a lot less money.
Very disappointed!