It’s vacuum-packed in a can like Spam. The external markings on Kul’s model indicate the bag was part of a U.S. Air Force contract. The contract number is difficult to run down, but it appears the sleeping bag was part of a contract for 1950s-era F-84F Thunderstreak jet fighters. That makes sense since the bag, sealed in a can, is clearly meant to be used only in emergencies.
Why would the U.S. military pack sleeping bags in metal cans? Goose down is compressible, but it’s also really difficult to get all the moisture out of a down bag if it gets wet (unlike modern nylon shells). If an emergency sleeping bag gets wet in storage you might not notice for years, and by the time you really, really need it you could be left with a mildewy mess. Thus the impenetrable metal can.