This guage is used to time your rainmaker. Sometimes the factory settings are off, which causes your rainmaker to skip shots at higher rof. The guage is cut the the exact length for timing your rainmaker. The guage saves you the trouble and high cost of getting a set of calipers.
The Bad Mojo Werks Timing Gauge is older, so while it may be available used and in a few cases new, it is not commonly available anymore.