New Standard Motor Products T-Series water temperature sensor.  This is for factory gauges (with a coolant temperature gauge, no idiot lights).  1/2" NPT.  Fits the following:

70-78 Firebird/Trans Am w/ Pontiac 326/350/400/355 engine
77-78 Firebird/Trans Am w/ Olds 403

NOTE : Early model engines used a button style sensor end, while later model engines used a tab sensor end (See photo #2)