The ring is probably a cocking knob. I have a Marlin single shot that has a "T" where the ring is on the one you are describing. The knob has to be cocked much like the hammer on a single action revolver. If you cock the rifle and dont wish to shoot, you can let the knob down like you would on a revolver.
My little Marlin is a very accurate gun. Depending on the price and condition of the one you are looking at I would say buy it.
If it is in good condition, I would pay maybe fifty bucks for it. I got mine for twenty bucks but it was in bad shape.
The Marlin is the one in the middle. The one on the bottom is also a single shot Stevens that has a ring like the one you describe. I gave forty bucks for that one.
