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

78-81 Firebird/Trans Am w/ Pontiac 265/301/400 engine (81 models with 1/2" NPT only)
79 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)