It's a model 5100 Stevens.
311 Stevens parts might fit but there's also a chance they won't or you'll have to do some fitting.
I thought the 311 replaced it just before WW2, but they are the most confusing bunch of models and numbers of any I can think of.
When they appeared and when the disappeared don't really effect me much.
I would just try to keep the basic types sorted by looking at the side of the frame at the pins so I could ID them right for repair.
I mostly avoid them now but an 80's production 20ga 311 may make an appearance in the 'Sow's Ear to Silk Purse' catagory one of these days. My God what a club!!
