The green mask was known as a 'med' mask as it was better for people who experienced an alergic reaction to the black rubber. As the good news spread the docs were quite happy to authorise the med mask in lieu of the black beast.
Probably it became cheaper to provide the med mask formula to every one.
The thing that made the P & Q mask work was its small size and turned back face piece. The latter sat flat on your face, inside the mask. Overpressure would then ensure it sat tightly on the face.
The American attempt at a pressure mask was a huge rubber monster, the M mask, where the flange was outside the mask and needed to be held in place more securely that the P/Q.
The British non-pressure mask was the H-mask, a green outer and a rough white inner with broad grey elastic straps to fasten it to the G-helmet. An official mod on the H-mask was to thread a copper wire over the nose bridge to allow it to fit better.
The med mask was used to replace the H-mask.
The P/Q Mk 2 was secured with a simple metal chain. For the F4, with higher ejection speeds, the chain was changed to a bicycle type chain but this was a sod and prone to a 'twist-lock' which made it very awkward to put on as you had to untangle it first. In this guise I seem to remember it was a Mk 7.
Later, and I believe in its current iteration, the bike chain is encased in a plastic sleeve.
Not bad for a bit of kit introduced over 50 years ago.