If you not sure if it's the same tach I'd take your old one with when purchasing
They may have sold you the wrong style (Flywheel vs Alternator).
Which do you have? Alt or FW driven?
If you have the FW driven then there should be a sender, on the starter side of the motor
, screwed into the bell housing right over the FW. But most Yanmars I've seen have the alt driven tachs.
Some flywheel actuated tachs have a switch on the back for the proper number of teeth.