Ah, yes, the Elkhart Conn slide stocking wear problem! Many of these era Conn slides had this "trademark". If the spot is not too large, a good brass tech can possibly buff the spot and give you a little smoother slide action, for a while, at least.
The only other alternative is to have a brass technician, who is really good with trombone slides, put new inner tubes on this slide. This will restore the slide to like-new condition, if the slide is put back together correctly. Someone like our OTJ Forum mod, John Sandhagen can give you better advice and can actually do this kind of repair work. I really like to use brass techs, who are actually trombonists, as they really know what a good slide feels like, and appreciate what a good slide can do for ANY trombonist, regardless of their playing ability.