"compliments of the season" by o henry. -----------------------In "Pigeon" McCarthy's pocket was a section of one-inch gas-pipe eight inches...

In putting these two things together, the author is trying
to tell us in a humorous way that One-ear Mike is a criminal and that he has come from a
family of criminals.

Brass knuckles are, of course, used
mainly by criminals.  There is not much of a legal use for something like that.  When
the author says that they are an heirloom, he is implying that Mike's set of brass
knuckles has been handed down to him from his ancestors.  This implies that Mike's
ancestors were criminals just like he is.