I think S = Straight and R = Reflected. So the R's bounce their illumination onto the back reflector in the headlamp and thence out the front. While I guess straight just bung it straight out the front, with a lens to focus it.
I don't think they are interchangeable.
I investigated importing these when I paid £105 to replace one in an Omega back in 2001/2
Thankfully prices have since dropped
