The spring loaded alloy clothing clip is equally fine and also stamped REF 6D/ 526 MK IV.
RAF MkVIII Flying Goggles Strap Buckle Helmet Oxygen Mask Spitfire WW2
Raf h type oxygen mask. 46 dated.
Vintage RAF C type Flying Helmet. Mark VIII Flying Googles & Type H Oxygen Mask
Original 1966 RAF G Type Flying Helmet & Type P Oxygen Mask Cold War Period
Unissued Q-Type Oxygen Mask Rubber for RAF Flying / Flight Helmets (171)
New Q-Type ( Medium ) Oxygen Mask Rubber for RAF Flying / Flight Helmets (151)
