I never had a problem mounting threaded carts on my RS-1A. I used a Te Kaitora and a Shelter 901. As long as you can put the screw up from the bottom of the cart, it will work. This rules out wooden Grados and Kontrapunkt/Jubilee, for example, with mounting holes that don't go all the way through the cart body.
Good compatible carts for RS Labs A-1 tonearm?
I just finished my Lenco 78 slatedeck and have an RS-A1 tonearm ready. I've heard of issues fitting some cartridges. Any suggestions on reasonably priced mm or high output mc carts that align well with this tonearm would be greatly appreciated.