Those are standard markings for all Marbles-Goss receiver sights. The Goss was not std. on your Stevens, so it's been upgraded from what Stevens shipped them with.
Your M-G sight should have a slide bar on the rear edge that is held with a single set screw. This slide bar can be zeroed once you've got your gun zeroed for 100 yds. If you don't plan to shoot your gun that distance, then the marks wont ever be employed. I'd guess most .22's don't get shot that far, so most shooters ignored these 100 and 200 yd. markings.
Anyway, once your gun is on target at 100 yds. or 200 yds (whichever you shoot) then the slide can be loosened and either the top pointer, or bottom pointer can be zeroed at the distance chosen. 
At this point you can adjust up or down, and always come back to the original zero you preset. As you can see there's no way to be on for both distances, and this was just a way to return you to close proximity of one of the two chosen distances.
I personally ignore th markings totally on my MG sight mounted on my Win. 52. I simply keep track of clicks in any direction to compensate for conditions, and then return it to original settings after shooting. A notebook helps a lot more at the line than these markings.
Hope this helps.