It's for cleaning rod, Pete.
My thouts are the following.
This DR was not boar shooting only, it's for big game such as moose, bison, deer etc. Probably it was heavy conical bullet of different weights and sliding sight helped to find POI at 500 yards at sliding sight position #1. Then moving sliding sight forward it was easy to find and to shoot at 600 yards, 700, 800...
But I can see one big problem here, moving sliding sight forwards the line of aim becomes shorter and poor acuuracy as result.
Guess it was interesting, but unfortunatly useless device.