Beam Approach System. I last flew it at Tern Hill in 1960. It consisted of an MF beacon that radiated along the approach path giving continuous 'A's when port of track and continuous 'N's when starboard. When on track they merged to a continuous note. There was an outer and inner marker which where passed at a certain height. IIRC these were 1,500 ft. and 100 ft..
There was a more refined version used in multis that had an instrument similar to an ILS indicator but these would not have been fitted to a Hurricane.
When used for the opposite runway then the 'A's and 'N's would be transposed.