The fuse box on early W123 cars, those with the glow plug relay inside the car and under the dash, usually 1980 and earlier, are located on the firewall back behind several of the soft rubber heater hoses. The location makes it difficult to photograph the fuse box when installed.
To uninstall the fuse box simply locate it on the firewall and remove the two Phillips head screws that hold it to the firewall. These are the Phillips screws located on the upper right and lower left corners of the fuse box, outside of the fuse box cover.
Once the fuse box is removed from the firewall it can be gently pulled forward for easier access to the screws holding the wires and fuse in place inside the box. As shown, once you've pulled the box out from under the coolant hoses you can remove the two remaining Phillips head screws and then remove the fuse box and old fuse.