Parts list: 1. Breathing tube 2. Cable guide 3. Hose clamp 4. Connector assembly 5. Nosepiece strap 6. Hook strap 7. Strap assembly 8. Catch 9. Adapter & buckle assembly 10. Nosepiece 11. Insert 12. Exhalation valve 13. Valve cover 14. Flapper valve 15. Band 16. Valve seat 17. Microphone plug 18. Face piece 19. Spring hook 20. Clincher tip 21. Screw

A reference dated 1970 is slightly different again. The A-13A mask and the MS22001 masks of the early 50s were essentially the same in terms of function, and certainly similar in all functional details except for items such as the suspension straps and connectors. Originally, the A-13 mask had a standard WWII type friction fit connector on the end of the hose. This was similar to the WWII Luftwaffe's 02-mask hose attachment system. It also had a standard constant flow system type connector fitted to a junction at the proximal end of the hose, between the face piece and hose. An emergency H-1 and H-2 type bailout bottle supplies could be attached. This was soon thereafter eliminated (late 40s) by a modification to combine both emergency bailout bottle hook-up and aircraft main 02 supply inlet in a single connector fitted to the end of the hose. The Navy's ACEL pioneered this type of connector and it was adopted by both services (this was the USN BUAER spec 83-C-3 connector, designated 191A2). The new system was found to be much better than anything previous used. When it was found that connectors of this type were sometimes pulling loose from the female 02 hook-up receptacles unbeknownst to pilots (and thereby causing potentially fatal LOC due to hypoxia situations), the design was modified with a disconnect warning valve device built into the connector. This resulted in the type MC-3 connector designation. All of these connectors featured a nylon snap-fitted strap that could be secured to prevent flail to some degree. Pictures of the different types of connectors may be found here.

A still further minor redesign of the emergency disconnect valve device resulted in the designation MC-3A (also known as the MS22016 connector). Although at first glance all of these connectors appeared similar to each other (there were variations in the colour of the anodised aluminium parts -- early 191A2 non-warning type connectors featured green anodisation, while later warning type connectors featured red or in some cases white anodisation). None of these early connectors incorporated suitable anti-flail features to keep hoses from inflicting windblast flail impacts on the pilots in emergency ejection situations (the nylon strap system was not always effective). This problem was rectified by development of system wherein an internal nylon cord connected the mask's distal facepiece connector to the distal hose connector; however this was a feature that only the MS22001 mask fitted with the new 3-pin connector had. The 3-pin connector attached to a chute harness connector which stayed with the pilot (this was the CRU-8/P, later replaced by the improved CRU-60/P, which incorporated both emergency disconnect valving and emergency bailout 02 hook-ups). Very early 3 pin.connectors, designated 57B3661, were machined from a single piece of bright aluminium and had squared off pins. A close examination will reveal this. Later versions, designated MS27796, featured round pins and refined machining techniques (as a result, these early bright aluminium 3-pin connectors are rarer and more valuable to a knowledgeable collector).

The addition of the 3-pin connector to the hose of a mask, the use of a different length hose, or the use of a different helmet-to-mask suspension strap arrangement, all resulted in redesignation by different specific identification and spec numbers. Thus there were the different A-13A, the MS22001, the MBU-3/P, etc., masks, although in every case the actual mask itself and its function remained essentially the same (except for incidentals, such as have just been referred to). It was also pointed out in a previous commentary that the A-13A and early MS22001 masks could be used with a standard one-way rubber check exhalation valve. As that used in the demand mask type A-14 and its variants. They are sometimes (although infrequently) still found with this type of exhalation valve fitted, instead of the positive pressure breathing exhalation valve that was used almost universally in masks of this type from the mid-50s onwards. It is suspected that the MS22001 mask was also used as an emergency demand type mask in certain aircraft, so this might also have resulted in the substitution of an ordinary one-way exhalation valve in an MS22001 or A-13A mask for the pressure-demand valve.

Note: "This article is tentative, and informal account, and as yet not complete; the information appearing here is not intended to serve as a final, definitive, or authoritative reference to the MS122001 or A-13A mask. A more refined, more complete, and more accurate version is required and may be available in future, upon accomplishment of cooperative collaboration with others (Principally Steve Norris, who is presently in the process of researching this and related areas of concern for his upcoming book on these systems)".