The P/Q series of masks (and other derivatives such as the X-Type and AR-5) have been the mainstay of the RAF and RN for a very long time, and alongside the H-Type is one of the longest serving types of oxygen mask. Most RAF / RN aircraft are still flown with variants of the P and Q-Types.
There are many different flavours of P-Type and Q-Type oxygen masks, to suit different aircraft and different roles. The variants are distinguished with numerical suffix, and sometimes additional letters.
I only have information on the types up to and through the cold war, not much knowledge on the currently in service variants.
Further variants of these masks have been exported to other countries and adapted for use with American helmets. Notably Canadian and Australian air forces, who operate aircraft with oxygen systems more suited to the P/Q-Types, than American masks.
The term "P/Q Mask" or "P/Q-Type" is often used to describe two very similar series of oxygen mask, so it's convenient to group them together.
The difference between The P-Types and the Q-Types is purely size, P being the larger, and Q being the smaller.
Why not just produce the P-Type in two sizes and call them "small" and "large"? - read quote below:
Quote:
The facepiece of the P mask did not provide a sufficiently good seal for pressure breathing on all faces, particularly smaller ones. Accordingly a new, smaller size was introduced. Because a different manufacturer was given the contract from the one who made the P mask (a policy of the Ministry of Supply to encourage competition to prevent the Ministry from becoming too dependent on one particular company), the new mask was called the Q mask.