as to versions of the 135 -- version one, I believe, used a series 7 filter, version 2 used a 55mm (I think) filter. Other than that, they are pretty much the same. The Leica Pocket Book says the version 2 has a different optical design, but the test results, and description, are pretty much identical. I have an earlier version.
Excellent lens. Built like a tank. If you have an R8 you may need one that has that ROM coding on its mount, but I think they'll all work. Leica only made this one 135mm lens, and made the same lens for 30-plus years, the book says, and if Leica couldn't find a way to improve it, I'd say it's pretty good.