Reading your updates, my first question is why do you think it is 120v and not 24v?

I have had forced hot water in three houses (built in the 40s, 80s & 90s) and all three used 24v for their original boiler (though the one from the 40s was converted from steam to hot water at some point using the same boiler).

If you have access to a multi-meter, that will give you a definitive answer, just make sure it is in AC mode (not DC).