CTC 43658 Remote Indoor Temperature Sensor.  For use with CTC thermostat models 43058 and 43558

Wire with 2 conductor thermostat wire
Measurement range 15 - 99 degrees F
Accuracy +/- 1 degree F
Maximum connection distance - 328 ft. with 20 gauge 2-conductor wire

3-1/4" X 2-1/2" H X 5/8"