S,L, LR means Short, Long, or Long Rifle. Means other than 22 Mag, it is pretty versatile on ammo.
Myself, would run a brush and patch thru the bore- feeling for rough spots or bulges- check safety and trigger, take it to the range, be sure you have glasses and muffs, and shoot it. For semi autos, first firing, I only load 3 rounds for the first time. Have had a couple of BRRRRRRRRRRRRRRRRP experiences (back to the seller, thanx!)
Nice little Stevens.