Does the 13 signify their affiliation with the Mexican mafia gang?
Ask somebody else what the "13" means to them. I'm not into lucky numbers, or unlucky ones either way although I recall a Gulf Shrimper from the 1970's that was called Lucky Seven and thought that a pretty good name for a shrimp boat. Then there was that tv commercial where heaven was "oh thank"-ed for 7 and 11, the Southland Corp having been a growing concern in those days, having stores in sight of one another in some places, all this I'm talking 'bout a little before Big Gulps made their first splash across the small screen and parking lots near you.
To possibly help make things easier, if that number is bothering you, and to provide an answer more narrowly addressed to the question you posed, I can tell you I was seeing "13" around (in Texas) long ago (like 50 years ago) before it later began be associated with mez'kin drug gangsters.
Figure out what "13" may have meant in the years from about 1965 to 1975, to a Texan, in Texas, then go on from about there.
Does that make sense to you? Answer that question, Princess, or we'll just have to give up dating each other for a while, "see other people" and all that...
It's not you, it's me. Screw that. It is you. Answer the question.