USMC Officer’s Field Pack.(second/later version) Similar in design to an M1936 Musette but with an integrated shoulder strap that converts to backpack style with a snap on the bottom. The size is a bit bigger than both the Musette and the M1941 haversack. The internal divider is also set in the middle of the pack vs the the very back as on the Musette and also there is no pocket on the back.
I should have put an M1936 next to it as it’s side profile is less pear shaped making it deeper as well.
Here is a side by side comparison with the typical M1936 Musette bag on the right. As you can see the USMC bag is noticeably longer.
See the Iwo Jima post for more pictures