Can anyone tell me what this is:

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It's on one of the hot water baseboard radiators in my girlfriend's home. It can be turned one half of a turn with very little resistance. Someone suggested it's a thermostat, but this is a single zone system that has a central thermostat that controls the pump and boiler, so I can't imagine how a baseboard-mounted thermostat would work in this system. I admit that I am new to hot water heating systems though.

Any ideas?