Be advised used backs can have spacing problems.
The ratchet advance usually can be relubed and there is a how to on the net but the Rapid back I tried to fix had a slightly different configuration.
I think 2 out of the 3 backs I have don't space correctly.
You probably know this but thought I'd mention it.
Good luck with your search. The Koni's a cool system.
oh btw: the 135 is rare and commands exorbitant prices.
It wont focus all that close either.