What's inside a metal stuff sack.

The US military used to store down sleeping bags in waterproof sardine tins for use in emergency situations. From Popular Mechanics:

The M1949, also known as “Sleeping Bag, Mountain, M1949” is well known in collector circles. Introduced in 1949, the M1949 is what we would today call a “mummy bag.” The bag features a cotton shell with down filler and is meant for mountain or Arctic conditions.