The recurrence of this combination of numbers in Montana is fascinating. Supposedly, it started in the days of the Montana Vigilantes to give the unlawful element (called "road agents" in the parlance of the time) a clear warning to leave town or face the consequences. So what is the meaning of this egnimatic group of numbers? The most widely accepted view is that the numbers represent the dimensions of a grave: 3 feet wide, 7 feet deep, and 77 inches (6 feet 5 inches) long. Alternatively, the “road agent” was given a specified amount of time (3 hours, 7 minutes, 77 seconds) to leave town. Otherwise he would be a grave man. It may, however, be something of Masonic significance.
My theory is just that grown men managed to include some horsing around in what was definitely serious business. Guys love secrets; especially weird ones. This is what adds to membership in the Masons and other organizations with rituals. The Ku Klux Klan did pretty much the same. They did malevolent and illegal things, but managed to have a little dress-up fun and quaint rituals in the process.
After all, how many people know the actual dimensions of a grave? And did watches in the 1860's or 1870's have second hands?
By the way, the Montana State Troopers have the 3-7-77 on their shoulder patches, to indicate that they are serious!