Mark, we are looking at a raleigh frame, after the TI take over, when they owned about 100 names, some for export markets only. Raleigh used to drill and pin the lugs to hold the tubes in place for brazing. I think Phillips/TI (as BSA did previously) used the cheaper method as seen under your bottom bracket, bang a large dent in to hold the tube. So this is probably not long after the large take-over by TI.
Wiki is wrong about BSA buying Triumph cycles, it was Triumph motorcycles, a totally different company. Raleigh bought Triumph cycles.
Not a lot of use to you, but not many of us left to remember. I went round the BSA cycle factory about 1955.
Your bottom bracket threads look like raleigh 26 tpi.
Keith