Seems like one of the Baptys from Prop Store indeed. But I'd really want to know if it really is that. It uses some narrower t-tracks (Marv version maybe?) since you can see the barrel holes and the cylinders are pretty good (PWC maybe?). We know Bapty stripped the Sterlings after SW filming was over, so everything on it was added later. So the cylinders and t-tracks are not screen used, neither original found items, but replicas.
M40 scope, pretty nice and accurate. Eagle Hengstler and scope rail - cool as well. Maybe there was already a hole in the back of the Sterling from the previous conversion, thus just to add a scope rail and screw in there? The trigger guard/screen used connection photo is a bit stretched though.
BTW just realized it comes with 2 different scopes, look at the feet.