There were two basic models of these Stevens, which had a generally similar appearance externally, but a different mechanism internally. One had internal swinging hammers while the other was striker fired. The 5100/311 had the internal hammers & were essentially the same gun, except as I recall the 5100 was normally better inished than the 311. The Striker fired model was a 315 & will usually have a 1915 patent date on it.
If I haven't got it reversed thr sear pins are the rear ones & on the hammer style they are lower than the front pins, down near the bottom of the frame. On the striker fired model the rear pin is higher than the front pin.
