Not mysterious at all as it happens and in my experience a very common sight on mid-range mass-produced (not RSP) 531 Raleighs of the 1970s-80s...
This is just a 'stop' to stop the clamp-on levers from sliding down the tube when they are used (usually if they were not tightened enough on assembly).
You'll notice the 'pointy'-end on the braze-on fits nicely into the split of the clamp-on band (well it does on my Raleigh winter-hack anyway - which is fitted incidentally, with my vote for the best changer levers ever made incidentally - Sun Tour Ratchet Frictions - a bit chunky but wonderful - and still appearing regularly on an e-bay near you!).
Ian Briggs
Luton - England