Philippe,
1939 was the last year for bulb/reflector type headlamps on a Chevy. The truck harness would not have used a junction block because the wires would have reached the lamp with a 3 contact plug which was a push and twist into the headlamp bucket. There is no application listed for a terminal block for a '39 truck in my '41 Master Parts list. The earliest truck listing is for 1940.
Someone with a 1940 truck can tell you where the block attaches.
Mike
