The script writers for the Snafu cartoons were typically uncredited, though animation historians consider that several of them were written or co-written by Dr. Seuss and Munro Leaf.
When Snafu opens the case, he reaches in and retrieves first a sheer bra, then Bugs Bunny, and last his mask.
[1] As Snafu relaxes under the tree, he comments on "the smell of new-mown hay, apple blossoms, flypaper..." whereupon he finally realizes that he is in danger of being gassed.
[1] The short was part of the ongoing efforts of the military to convince soldiers that their gas masks were more than "dead weight".
As the war progressed and the expected chemical warfare did not occur, soldiers were increasingly likely to view both the masks and their training in using them as essentially useless.