Message 3435,  Jul 14, 2001
MichaelFox, The original A-14 (1944ish) was used with everything from USAAF WWII era helmets through the first hardshell (P-1) helmet (for non pressure-demand requirements). The subsequent A-14A, which was only a very slightly modified version of the A-14 oxygen mask, was used with any aircraft that did not require pressure-demand breathing (limit was about 30K feet); this meant that it "could" be used with all the P-series helmets at least up until about 10/67 (that is the manufacture dated I have for a specific example of the A-14A in my collection), again in the same (lower and slower) flight operations envelope.
The still further modified A-14B and MBU-4/P masks could also be "legitimately" used with P-series helmets, all other soft or semi-rigid helmets, and even with the HGU-2/P helmet (up to about 30,000 feet altitude). There were instances in which the MS22001 mask was used with a standard demand exhalation valve (non pressure demand valve made of rubber--identical to that used in the A-14 series masks), with inlet check valves removed, for low and slow flying (certain types of multi-engine aircraft included, due to the lack of adequate room the A-14 series masks had for later microphones). [In other words, a ordinary MS22001 mask thus modified could substitute for the A-14 type masks in "low & slow" flight.
Again, the last two masks (A-14B and MBU-4/P variant) could be used with any helmet for low altitude operations). The only qualifier technically would be any limitations imposed by LS equipment TOs on these uses and applications, and certainly a late manufactured (1966) A-14A could not realistically be used with a P-1A, since by that time the P-series helmets had been essentially replaced in their entirety by the "new" HGU-2/P helmet and P-1A types had long since been superseded.
Essentially simplified, the rule of thumb would be that A-14 types are for lower altitude operations, not requiring the pressure-demand system that higher altitudes (about 30K feet and above) mandate; all higher altitude flights required use of the MS22001 (or MBU-3/P), then later the MBU-5/P, MBU-12/P, and MBU-20/P masks (all of these are, of course, pressure-demand systems).
If I were matching up masks with helmets for absolutely authentic time-frames, in terms of display, I would be careful to use masks with a date of manufacture relatively close to that of the helmet, although some latitude is permitted (an earlier mask can be used with a later helmet to a certain degree, but not reasonably the other way round).
The absolute limits on this matching compatibility for the complete obsessive-compulsive stickler would be the mandates levied on applications of use by the pertinent TOs (helmet and mask).
Cheers, DocBoink