I could be the clip that holds the capillary tube coil to the heater core? This clip is discussed and location shown in the 1963 Oldsmobile Service Manual. I do know this capillary tube is filled with a liquid but it is not mercury, most likely alcohol/water.The fluid will expand inside the capillary tube with heat from the heater core and help maintain the inside temperature of the car to remain somewhat constant. In my heater core pic, I do not have the capillary tube coil located in the exact location as shown in the manual, but this is not so critical. Pics attached
