Nothing unusual in this... as George pointed out, this was the standard seat stay cap treatment on Raleigh SBDU Reynolds 531 machines in 74-75 (maybe later still) as were the gear lever bosses used as water bottle bosses. Even odder, they didn't use gear lever bosses as, well, gear lever bosses on these and clamp-on gear levers instead. Go figure.
Peter Kohler
Washington DC USA