You don't state whether or not your Gibson is electric, but it probably is, so does it have a separate selector switch for bake or broil? Often the highest setting was to prevent the broil unit from cycling during broiling operations. A friend's GE wall oven from around 1960 had "BROIL" at the top end of the thermostat dial and it was 550F but the broil element was not activated unless the Oven Set dial was turned to BROIL. Old Wilcoltor thermostats like on my 1954 Frigidaire RT70 click into the broil setting above 575F. I think this was not only to prevent cycling during Broil, but also to accommodate the mechanics of the thermostat that allowed the switch from powering the bake elements to powering the broil elements.