Hi T6flyer,

There is an Air Publication held by the RAF Museum Hendon that contains details of all GGS types. Call “D.O.R.I.S.” – the Department Of Research and Information Services. There are a few pages on the MK4B variant you are enquiring about. Sorry, I can’t remember the AP reference number, but I found it there by simply asking for their “RAF Gunsite AP”.

Hope this helps.