The S-Type is a bit of a mystery mask.
Some say it is the designation of the special P2/Q2 mask modified for the BAC TSR2 aircraft, but I'm yet to find any evidence of this. the only evidence I have found is below:
Quote:
The P and Q facepieces have proved the basis for all recently introduced RAF oxygen masks, different marks being accorded different suffixes depending on the accessories supplied with the mask, and derivations of the basic design being called S, T, and V.