I got mine from Old Britts. There was a long thread on the different types of iso's out there. Seems like at some point, maybe the 850 they switched to a harder rubber. Grandpaul has a vernier setup, but it is a different style than the early 750, same as Norvil. It changes out all your old stuff. Phil at Fair Spares has the old style soft rubber bushes too. I didn't want to change mine so I used the old style rubbers and the shims. At some point I will probably get Hemmings vernier adjuster. I think the most reasonable priced ones are OB parts and Hemmings adjuster and you don't have to machine anything. Don't know if RGM has the old style sets, but they have good prices too, even with the shipping.
Dave
69S