Dave -
The black connector is probably a bullet type wire connector. I don't think that heaters had in-line fuses, unless added by an owner.
Does it look like the one in this "before" picture of my '54 Air Flow heater? Bottom wire is the hot wire with bullet connector. The other wire is the ground.
> '54 HeaterAIR FLOW HEATER:
See Figure 100 in the link below. Look for the word "CONNECTOR" near the back of the valve cover. Follow the indicator line to the connector on the wire to the heater.
> Figure 100RECIRCULATING HEATER:
See Figure 105 in the link below. Look for the words "MOTOR WIRE CONNECTOR" in the top center of the diagram. Follow the indicator line to the connector.
> Figure 105> BULLET CONNECTORS (Google images) available at most auto parts and electronic supply stores.