Sometimes there is a spare wire that isn't needed that is part of the conductors in the cabling. Mine has this too. If its not used now, it probably won't be needed later.

It appears this is a L7224U Oil Electronic Aquastat controller. I found the schematic but unfortunately there is no labeled C (ground) connection. The red and white wire near the top go to the thermostat. Even the R terminal (24 V) is not labeled. It is important that is verified before connecting to a WiFi thermostat. The polarity doesn't matter on a simple two wire thermostat, but if you wire it backwards on a electronic thermostat you could damage it.

There are some Nest models which claim to work without a C wire, but no guarantee it will. The best option would be to have a separate power supply.

Look at figure 4 of the installation instructions. The two terminals to which you connect a thermostat are at the center-top of the drawing; they are not the row of screws in your photograph. Your system alone supports only a simple, two-wire thermostat (example: Honeywell CT50A).

-- amateur