I have Bruce Canfields book, "A Collectors Guide To US Military Shotguns", and it has some pics of the pintle mounted aerial gunnery shotguns.
Doing a bit more reading in the book (its been a while since I picked it up), I've found that the shells are probably packed in 12-10rd boxes inside the can, giving it the 120 cartridges count. Three of these cans like I have "M10" were put into one "M12" wooden packing box sealed with wingnuts. You can see the pictures in TM9-1990 "Small Arms Ammunition
I am 90% certain that these a paper hull shells.
According to the book, printed in 1992, "A collector would be well advised to purchase such ammunition in the original cartons when found at anywhere near reasonable prices since the demand and cost will certainly continue to escalate".
I am now starting to consider holding off and throwing it back up in the attic.
SUCK IT, TREBEK!