BSA Gold Stars are very desirable hand-built racers.  (Catalogue: “The Clubman’s model Gold Star has been developed for competitions in road and short circuit events, and its specification is such that it is neither intended nor suitable for road use as a touring motorcycle.”)  Some consider it the most successful BSA, running from the 30s to 1963.  Their single cylinder 499 cc four stoke engines produced 42 hp at 7000 rpm, coupled with low weight, 380 lbs., assured that these were very fast, successful racers. I guess that this bike was made between 1954 to 1963.  Here is Revival’s page on this bike, and good history here.