I've previously heard and seen(prior to seeing this) about the lousy quality of some 70s Raleighs and it was brought up again in this thread:
Here is some insight(sorry for the pun) of inside the brazed lug. I cut up old frames to practice brazing, when I cut up a 1973 Grand Sport initially thought wtf, some worker lost his wedding ring in the top tube? No such luck, it wasn't gold but a brass brazing ring, maybe from hearth brazing process(don't know but guessing).
The frame had failed at the seat tube/top tube lug due to penetration of water at the lug. Unfortunately I already tossed that part of the frame so I couldn't inspect the lug when I discovered the brass ring lodged in the tt/ht lug. I'm guessing the ring was installed in the st/tt area and never melted, then slid down tt and has ben in there for 40 yrs.
I saved the ht and the other day when I picked it up the ring fell out, discolored from the rust in the lug but it shined right up. The ht/tt and ht/dt lug pics both show partially melted brazing rings, maybe enough mtl flowed in the lug to make it sound but I doubt it as the other end at the st wasn't.
Sort of confusing to talk about, here are some pics. Brian