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This is the same older style Honeywell Triple Aquastat relay with the cover removed
The Hi setting is the high water the boiler is set at 180 degrees F is the average setting
The Lo is the low setting this is the setting that turns the boiler on 20 degrees lower then Hi is normal
The square thing on the right is a transformer that takes the power from 120 volts to 24 volts but beware the box has 120 volts inside of it. just under the transformer is the relay.  

Honeywell open aquastat