C-Wire Williamson gsa-125-n-ip
The soldered C terminal is labeled on the transformer board. To confirm you should be able to follow that wire to a ground terminal. You might have to cut and splice that wire.
That being I'm not convinced from the blurry diagrams that the standard furnace wiring setup will work with your boiler, after connecting the C wire you may need to connect the R from the boiler transformer board to the Rc terminal on the nest and use the Rh/W1 on the nest as dry contacts for the two wires currently connected to the old stat.