The purpose of the UV filter is to reduce distant haze although its main purpose is to protect the front element. The Skylight filter was often confused with the UV but its use was to reduce the blue effect of open shade, overcast days, snow etc.
The close up lenses have the prefix S for the 24mm lens, W for the 18mm and T for the 50mm The number is the focal length, i.e. S31 is 310mm.