this is the kind of workmanship that makes me wonder why anyone would collect Ralieghs, other than as an example of poor workmanship. I don't really understand their level of quality control in those days, it was just ridiculous. Not only was the joint not fully brazed, but they apparently forced the seat stay on and broke the tube. This would almost be forgivable if they had then filled the joint with brazing material
You didn't show us the braze at the front of the dropout, if it is solid I wouldn't worry about it. I wouldn't use the bike to carry a load, who knows what else is wrong with it.