SxS Double Barrel Springfield

What is the difference between a model 311 and a 5100? The one I have is marked Springfield Savage (I think) and marked 5100 on the frame. I have seen them marked Savage Stevens, I'm pretty sure the one I have is Springfield Savage. It has two triggers and a Plastic (Tenite) checkered forearm. The Buttstock looks like a replacement, possibly home made. The rear of the frame has a notch on both sides compared to being straight up and down.