The "P", I suspect, is an inspection stamp. I have an M8 with the "P" accompanied by and oprdnace cross cannons stamp. "P" might signify Proof. Something,like every 25-100 pistols off the assembly line would be inspected by the resident inspector for quality control, then marked. This would explain why only some have the "P" stamp and many do not. The ones with the "P" would be the ones physically inspected while the others were just boxed and shipped.
Cutiger, the pistol is 37mm and only made in that gauge. The 40mm reference you found was incorrect and an oversight on that authors behalf.
The Mount M1 or port is technically an accessory for the M8 pistol but more accurately Aircraft Equipment. They were intended to be riveted into the fuselage making it a permanent feature. This is why they are very hard to source as they were not intended to be removable or transportable. After its adoption, the M1 mount was installed at the aircraft factory before delivery and was integrated into the standard drawings for each aircraft that were authorized to have one. Mounts were procured for supply and were directed to be installed in those aircraft that were already in service and delivered without it from the factory, essentially retrofitted in to bring all aircraft up to new standards. There were those that were in inventory as replacements if necessary. Another way to look at is that every M8 pistol included the mount, but the mount was permanently attached to the aircraft and there it stayed. Upon decommissioning, the pistol was removed and the mount was recycled with aircraft. Pistols salvaged and the mount destroyed, hence the discrepancy in the ratio of availability. Plus! the mount serves not practical purpose outside of use of an airplane. All those pistols installed for alternate applications, the mount was discarded. There was a heavy consumption of the mount M1, all USAAF bombardment and transport aircraft used them, all Naval aircraft used them except fighters or VF-Class. However, the mount was used in USAAF pursuit and Navy VF class aircraft but in a limited capacity. for awhile, those fighters used a Molin discharger instead.
Thorin, the jury is still out on that leather holster. Though it is obvious it was molded to house the M8 I have found Zero record of it in US Government literature, at least WWII. I've looked through all sorts of both Army Ord. and Naval Ord. manuals and stock catalogs and never once is the leather holster listed. Some examples can be found with MADE IN CANADA ink stamped on them. I question, at this time, if they are even US. I can prove how they carried and what was approved for carrying the M8 during WWII but again, no record of the leather holster. I would love to see some Government literature picturing or mentioning this leather composite holster, no matter what era. I've tried to look but with no luck.
This is another item that we've been lead to believe for many decades to be a WWII item in which many have in their collections but yet there is no supporting evidence as such, only we know it exists.
An important thing to remember and consider in regards to the M8 was that when in the mount, attached to the fuselage, you can reload and fire it without its removal. Its principle storage was actually in the mount itself. There was a canvas bag for its storage during WWII when not secured in the mount. That leather holster serves no real purpose but certainly had an intention for some application. I think the biggest clue and guides us to look is the Made in Canada stamp. The factor to consider is that these pistols remained in service for many years after WWII, decades in fact. I lean towards the belief that these leather holsters have some jet-age application. Additionally, the reinforced aluminum plate on one side plays an integral role in its application.