One of these is seized absolutely solid on my Model 10, and no amount of heat, penetrating oil and (judicious) brute force will shift it. I think I'll need to remove it with a nut splitter or careful use of a Dremel as I don't want to damage the thread on the stop itself.
Anybody got one lying about? I would think they'll be common to other models being 5/8" diameter x 20tpi, and very thin (around 3/16 thick). Patina no problem!
I think I've discovered that Triumph part 37-1798 is a thin 5/8" x 20 tpi component so this will probably get me out of trouble. Although it may have an AF spanner size.