OK, I have to add my $.02 and disagree with the group. I don't believe the M3 ammo chest had ANYTHING to do with aircraft. I think their original intended purpose was for Cavalry use and the clips on the rear were for use with pack saddle hangers.

Typically, I couldn't find the Rock Island pub on Cavalry equipment in the den last night, but did find one confirming reference. In the August, 1942 edition of the SNL A-39 on the Browning .50 M2HB, there are entries for the M2 spare parts chest, and both the M3 and M17 ammunition chests. In the footnotes at the bottom of the page, quantities of issue for the M3 chest are listed as....."Sufficient for ammunition authorized in the several Tables of Basic Allowances." The footnoted entry for the newer steel M17 chest says...."may be issued in lieu of M3 type for all units except Cavalry." So, apparently for Cavalry units, the M3 chest was still the preferred chest of issue.

As a side note, if you never noticed it before in the FM, the M2 spare parts chest was also supposed to carry a 40-rd belt of .50 ammo in addition to the various spare parts. That requirement was also made with the pre-war metal spare parts box used with the M1919A4 and M1919A2 machine guns. They also carried a shortened length of cloth belted ammunition, 100 rounds if I recall correctly without looking it up. I suppose it was partly for balance of the spare parts chests but I always thought it was cool to have that last ditch emergency ammo tucked away for those tight spots.

Here's some updated information for this thread. I finally located the Cavalry reference publication in my den/paper dump. It is titled "Notes on the Pack Equipment Cal. .30 & Cal. .50", and printed in January, 1937 at Rock Island Arsenal.

There is a lot of very interesting and obscure data in this pub including info on the M1922 machine rifle and the stamped sheet metal feet used with the M1917/M1917A1 tripods in soft ground. Ever seen any of those out there?

Regarding the M3 ammunition chest, the normal pack load for the .50 M2 HB included 4 of these carrying 120 rounds each, for a total pack load of 480 rounds, not including the 40 additional cartridges carried in the M2 spare parts chest. These are carried on the pack saddle with the aid of the "Hanger, Ammunition, Cal. .50, M9", dating from its June 1, 1934 drawing. Also included in the pamphlet is the original ordnance drawing for the M3 chest, dated June 1, 1934 as well.

A close look at the drawing for the M3 chest is pretty informative too and has led me to some new conclusions. I always assumed the two clips on the back of the chest were intended for use with the hanger to hang the box on the saddle. But there are no hooks or similar provisions in the illustration of the M9 hanger and, more to the point, the metal protrusions on the back of the chest are referred to as "spacers" in the M3 drawing. So, it finally dawned on me that they act to keep the box pushed forward in the hanger so it can be opened to get ammo out without pinching the lid and hinge.

I've also pretty much arrived at the conclusion that the M3 ammunition chest was NOT just intended for Cavalry use but was likely the standard ammo box for Infantry as well. On drawings from this era, there is usually a small box noting "dwg pertains to" with an entry of the next item up the food chain, so to speak. In this case, the M3 chest pertained to ".50 BMG, M2 HB (flex)", without reference to the M9 hanger. So I assume the spacer feature was important when used with the Cavalry hanger, and just an incidental feature when hand carried. But as most Infantry equipped with .50 BMGs would have also been issued M5 machine gun hand carts at that time, the spacer bracket might have also been useful for opening ammo cans packed tightly in the cart.

One final bit of interesting info from this pub expressly for Craig Johnson.....regarding the two flap narrow barrel cover discussed recently. A drawing of this item is also included in the Cavalry Notes publication. Officially, it is the "COVER, MOD '18, FOR SPARE BARREL AND CLEANING ROD FOR VICKERS MCH. GUN, MOD '15, AND BROWNING MCH. GUN, MOD '17". So we were both correct and the cover carries both a spare barrel AND the cleaning rod for the M1917 gun. That drawing is dated April 20, 1918.

Original WW2 style wooden ammo crate for two rounds of 4.2 chemical mortar shells, HE, M3 with M6 Prop and M9 Fuze. Natural wood finish with grey and blue painted ends and black ink stamped markings, brown painted steel hinges and hasps. Well marked on left end with the ammo nomenclature, black Chemical Warfare Service stamp on front, and paper label on back with nomenclature and rework date September 1947 from Pine Bluff Arsenal, Arkansas.