AiM TR (Thermo Resistor) sensor for measuring fluid temps. Is threaded 1/8-27 NPT for easy installation in most applications.
Also known as a PT100 sensor.
This is the newer style which features a small probe on the end of the sensor for better temperature gathering.
UPDATE: New part number X05NPTA4513BPRS vs old part number 3CVTRRNPT2K