A7 / 391 is the drawing number for a 1933 250cc head. For 1934 the drawing number is A7 / 391A. So what the JSB means I have no idea!
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), 54 4G Mk IV (project), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser.