Hey Av8tor,
its a TTO tach. I got mine from here( www.dasboata.com) I believe they can be purchased from motorcycle shops aswell. Simply wrap the signal wire around the plug lead. tacho can then be mounted anywhere on the model. A lot of the boat guys use them, once you shut the engine down it will display max rpm for about 10 seconds then switch back to run hours when not in use.For tuning purposes it was the best $30 I ever spent, chasing that last few hundred rpm.