Most oxygen regulators used in military aircraft are installed as an integral part of the aircraft, for fighter types typically in one of the cockpit side consoles. As such they do not attract much attention in flight gear collector circles. A notable exception is the mini oxygen regulators pioneered by the US Navy in the mid-1950s and still used to this date. In the US Navy the miniature mask-mounted regulator was especially designed for use with aircraft that have ejection seats. The console-mounted regulator is normally used in large non-ejection seat equipped multi-place aircraft such as the E-2 and the P-3.

The first mini oxygen regulator was developed by the Firewel company for mounting directly on the oxygen mask face piece. Mounting the regulator directly on the mask gave four advantages. Firstly, placing the regulator directly next to the breathing space made the regulator highly responsive to changes in the pressure in the mask. It would open almost immediately when the pilot started breathing in. Secondly, leading oxygen at high pressure all the way to the mask removed the risk of contamination of the breathing oxygen. The high pressure would keep any noxious fumes out of the system. Thirdly, it removed the need for intermediate pressure reducing valves, making it possible to reduce the overall size of the regulator. Lastly, the high pressure made it possible to use a smaller and less clumsy oxygen line to facilitate its application and removal. Further explanation of how the mini regulator works may be found using the link at the top of the page.

Far left and mid-left: Mini oxygen regulators installed directly on the oxygen mask face piece with the high pressure hose pointing directly forward and downward. Centre left: Mini oxygen regulator installed directly on the oxygen mask face piece with the high pressure oxygen hose turned backwards over the pilot's left shoulder. Centre right: Close-up of the clamp holding the high pressure hose to the helmet. Mid-right and far right: Mini oxygen regulators installed between the mask oxygen hose and the high pressure hose. The regulator and comms plugs are visible in the photo mid-right whereas they are hidden inside a protective pouch in the photo far right. From left to right photos are © Bluelight, Andrea Salimbeti, Milehigh, Milehigh, Trey Turner and Tomcat.

The latter advantage turned out not to materialise in reality.  The hose was thinner alright, but it was also more stiff, causing problems especially for fighter pilots who moved their heads around in the cockpit during air-to-air combat. As an initial measure the oxygen hose was connected to the regulator using an angular piece of pipe, routing the oxygen hose backwards from the regulator and over the pilot's shoulder. Some helmets were even fitted with a small clamp to support the hose. The modification did not solve the problem altogether so instead the mini oxygen regulators were moved to a position between a standard softer, more pliable corrugated mask oxygen hose and the aircraft oxygen supply hose, some attached directly on the life preserver on a special mounting plate.

MIL-R-19121 regulators
The first series of mini oxygen regulators were made to specification MIL-R-19121. So far I have found evidence of regulators made by three different companies, Firewel, Bendix, and Robertshaw. Around 1955 Firewel developed the first miniaturized mask-mounted oxygen system for the US Navy pilots flying the A-3D Skywarrior, F-4D Skyray, and the F-8U Crusader. The company took out several patents covering the technology underlying the mini regulators, the first applications being handed in already in 1952.
The MIL-R-19121 specification was cancelled on 2 November 1972, superceded by MIL-R-81533 calling for the CRU-79/P. The MIL-R-19121 regulators continued in service for some years, several of them still being listed in a 1978 NAVAIR.

Bendix 29211 series

Far left: Bendix 29211-B1 regulator. Picture © mil98 Left, Right and far right: Bendix 29211-01 regulator. Pictures ©
arklight1.

Firewel F-1732

Firewel F2700

Robertshaw 226 series

CRU-79/P oxygen regulator (MIL-R-81533)
Updated version of the MIL-R-19121 regulators. The CRU-79/P specification, MIL-R-81533, was issued on 9 April 1968, updated to MIL-R-81533A on 30 August 1983. The spec was declared inactive for new design in 2001, heralding the beginning of the end for the CRU-79/P regulator series.

Bendix 29267 series
So far I have not seen any photos of the Bendix 29267 series of regulators but a Bendix type 29267-A1 is mentioned in NAVAIR on oxygen systems in both 1973 and 1978, and the drawing below was used in a 1991-dated flight surgeon manual. Note how similar it looks to the 29211 series.

CRU-82/P Oxygen Regulator.
The CRU-82/P Oxygen Regulator is mounted by a quick disconnect attachment to the aircrew parachute harness. The regulator is an automatic pressure breathing type, providing breathing oxygen to the aircrewman's facemask on demand. It is designed specifically for the OBOGS and is characterized by the fact that it has the capacity to function at lower pressures than the 40 pounds per square inch (PSI) minimum required by standard miniature regulators.

CRU-88/P Oxygen Regulator
Vir prudens non.

CRU-103/P Demand Oxygen Regulator.
The CRU-103/P Demand Oxygen Regulator is designed for use with all tactical aircraft series oxygen masks as part of the oxygen system in aircraft with liquid oxygen systems or OBOGS. The regulator, which is manufactured by Carleton Technologies Inc., is designed to regulate oxygen to the aircrew member during flight. The CRU-103/P Demand Oxygen Regulator was developed as a part of the Navy Combat Edge system. It is a chest mounted, automatic positive pressure breathing type, Gravity Force (G)-modulated regulator that provides demand oxygen flow to the aircrew member including Pressure Breathing for Altitude and Pressure Breathing for Gs. This regulator is scheduled to replace the CRU-79/P, CRU-82/P, and CRU-88/P series regulators