I don't think that the 40A-R and 40A-L rings have a difference in sound reduction. You also have other options in 50A, and I believe I've seen 70A and 100A . 40A-R, 40A-L, and 50A-L can be found at WASD keyboards. You can also pickup soft-landing pads from Elitekeyboards. If you want, I wrote a review on o-rings and soft landing pads
here. You can check out more of my thoughts there. And for the record, my FC700R with soft landing pads feels amazing.