hdGuy,
I would venture to say that the main reason the Factory will not sell some parts to the public is because they require fitting.........very precise fitting that the VAST MAJORITY of gun owners do not have the skills, know-how or tools to do. I am not saying that you don't......I personally don't know your particular situation, but I would assume the Factory would not want the liability of sending out parts that would require such precise care. I'm sure their legal department put the brakes on that years ago. Drop in parts ( for the most ) are still able to be ordered as far as I know,currently.
I just fit a new one to an old pre M36 and I was quite successful, but I inherited a huge box of new-old-stock parts when my Dad passed away in March. I have also been working on Smiths since 1979 and have all the necessary tools, supplies and the skills. Again, not saying you don't but their legal Dept. does not know either.