It is a capillary tube and the unit is a thermostatically operated valve with the cable determining the set point. This service counselor article on page 105 shows where the tube should be attached so it is not "just hanging" and subject to damage.
https://www.packardinfo.com/xoops/html/downloads/SC/SC-VOL21NO22.pdf One way to test is to move the wire cable adjust lever to about midway so as to open the valve some. If you can see it visually, great. Otherwise, you may have to fix something up to be able to blow thru the valve. Using a low heat hair dryer, carefully heat the capillary tube end a bit to around a fairly hot heater air temperature and the valve should close. Be careful not to heat so much as to damage or blow the bellows. The valve may not totally shut off the water or air when closed but pretty near.
I don't know if those are the typical Ranco valves used 51 on with just a different tube or is a different brand. If the Ranco, am surprised the rubber valve seal is still good & not leaking. Those later seals you can still get from Napa but not sure if they will fit an earlier model.
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