The RS lever is a bearing rather than a bush, also has the swing link ‘servo’.
I think the lever blades are a little different as well, the R’s are just pressed steel. I know the R’s have reputation for being a bit spongy because of the lever, but the RS/RSC/Ultimate’s have all been great.
There was someone on Ebay selling internals, but actually made of metal, the issue is the piston plastic swells and fouls the wall, so being made of metal should hopefully resolve that permanently.
That said I have a set that’s coming up 4 years old & have mostly been faultless. Got a sticky piston once in the Alps, but I left the bike out in 30+ sunshine whilst having lunch & it was stuck on. Popped it in the shade and poured a bit of cold water on it & no issues since.
I’m a real fan of the higher end Guides as a brake. They are the best i’ve ever used, simple to look after, easy to bleed, light, powerful & easy to integrate with other components.