The ends of the fork blades where the dropouts attach look unusual to me. I haven't seen them done like that before. They were done well and somebody spent some time finishing them. They appear to match the chainstay ends although they are hard to see in the picture of the rear dropouts. I don't think the fork is a replacement. The ends of the chainstay bridge were mitered to fit not smashed to fit like you'd see on a lower end bike. The seat lug and bottom bracket shell look to be pressed not cast. That would indicate lower end unless the frame is a good bit older, from before cast lugs became popular. What I can see of the brazing looks clean. Again, somebody spent some time on the frame. I think you have a very nice frame that is older than the brazons would indicate. I think the brazons were a later add on.