I'm just guessing but it could have been for wild animals as well, at some of the rural airfields. This wouldn't have been too uncommon. Air force crews were issued survival rifles as well for similar reasons. Probably the oddest case of that is the survival pistols that soyuz crew have been provided with. Initially they were given makarovs but later on got their own specially designed TP-82, after Alexei Leonov landed near a pack of wolves.At any rate it's a great movie if you're interested in that period. The pilots all wear guns, as air mail pilots were required in the US as well. In case somebody tried to rob the mail, I guess. In one part flying over a high mountain pass they put rubber tubes in their mouths to suck oxygen from a tank!
A short piece on the aerial photography: