Correction - I found another similar tach on Images, and it clearly shows the upper center stud being the 12v connection (as does yours). This means one of the lower studs is for ground, and the other is for the signal wire, not 12 volts positive. Unfortunately I can't detect any markings on this other tach even under extreme magnification, so I would suggest doing low voltage testing of the gauge to determine which is the correct post before "going live":
