In 1940 the Marine Corps adopted the Army M1936 pattern field "musette" bag and modified it for their use by adding padded shoulder straps to it. I've seen documentation showing it as an officer's pack. Powers & Co and New York Drop Cloth Co. made it in 1940 and 1941. Atlantic Products Co and Boyt Harness Co. made them in 1942.

In 1943 and 1944 they went to a new pattern made by BOYT and S FROEHLICH Co. The new pattern could be adapted to be worn as a back pack or over the shoulder bag.

Here's the early pattern dated 1942.

A first year of production USMC Officer's field pack made in 1940 by POWERS & COMPANY which also made USMC shelter halfs.

Below are a few examples of the M.C.-modified Field Bags which Greg mentioned in the first post. The 2 at the top are 1944 Boyt-made Bags; the one at lower right is a "BOYT -43-". The only really substantial difference between these are the color shades; the Bag at lower left, made by the Sam Froelich company, differs in other ways from the rest.

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This Bag, in my experience at least, is unusual for this particular model because it is constructed of pea-green canvas (though they may not be as rare as I think they are); the great majority of these I've seen have been the familiar mustard-O.D. Marine contract duck...

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A little trivia: one Bag dated '43, the other dated '44, both with the same contract numbers:

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Stamped "S. Froelich Co. 1943", this is an oddball: soft web flap straps with no metal end tips and cast buckles a good deal smaller than the normal Musette...

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Detail of the Froelich Bag; note the difference in the "U.S.M.C." stamp from the widely recognizable Boyt letters.

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3 "U.S.M.C." stamps...

'43 Boyt:

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'43 Froelich:

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This is from the 2-shaded '44 Boyt Bag...what the heck happened??
Just that wierd stamp being on a W.W.II Boyt product makes this collectible...
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The Lite '43 manufactured version also has soft straps and no metal tip.