Lyric discussion by IvoKent 

Cover art for Fifty-Mission Cap lyrics by Tragically Hip, The

This song's meaning actually goes beyond just the legend of Bill Barilko. The fifty-mission cap that Gord D. refers to is the hat worn by fighter pilots in the World Wars (the ones with the flaps over the ears). After a pilot had completed fifty bombing or dog-fight missions they had the option to return home. Therefore, their hat became a "fifty-mission cap" which they would wear proudly. Gord D. is probably singing about a pilot cap which he attempted to work in to look like an authentic fifty-mission cap, as well as keeping his favourite hockey card tucked underneath. Classic Gord Downie nostalgia piece, conjuring up memories of his Kingston childhood.

@IvoKent thanks for the explanation on what a "50 mission cap" actually is. This makes it clear as day