That's a support for the fuel line. There would have been a grommet in the hole, and the fuel line through the center of the grommet. According to the TRA guidelines, the change to this type (from the earlier flat strap type) happened at the same time as the switch to push-on fuel fittings at the carbs; but I've had several cars with the push-on fittings and the flat strap line support.
BTW, your thermostat cover is upside down.