79 250 2nd temp sensor

[79Square]

I am a newbie, and just got a 79 C10, I6 4.1 250. I'm having issues with the temperature gauge readings, and the gauge stays at the low 'C' level. I am replacing the thermostat and the temp sending unit(s), and noticed that there are two sensors. One at the thermostat housing (see pic 1) and one in the cylinder head (pic 2) I have removed both (pic 3). Can someone tell me what the small one is? and how to replace it. I have been to a couple of parts stores locally, as well as googled, and can not find it anywhere. The thermostat sensor is a 3/8 npt, and the smaller one appears to be a 1/8 npt. Also, they both are connected to green wires, and when I ground them, they both peg the temp gauge to full hot 'H'.

Thanks for you help.