I'm meeting now with Arthur, the grandson of the steamfitter who installed the heating system at 210 Main Street. If you look at the cornerstone picture above, you'll see three men in a row with glasses. He's the one in the middle, in the background.
He tells me that when the first boiler was delivered, it sat in the alley to the north. They came back the next day and it was gone. They discovered that it sank into "quicksand". Prior to the construction of the Oradell Dam, the river came up to where River Street is now. He's guessing the soil was much looser then. He believes the building is sitting on hundreds of creosote-soaked wooden piles.
Apparently, the boiler is still buried there.