ClassicCar

If you're asking about the "cross piece," upper center photo, that's a bracket for the under-the-running-board antenna wire, for the radio.

Here's a photo of the installation on my '36, from the front, looking rearward . . . you can see your bracket at the rear. There were two radios in '36 . . . the standard one had the "long wire" antenna only under the left running board. The deluxe radio had the antenna wire under the left RB, then crossing over to the right RB and weaving back and forth there. The crossover was at the rear, not far forward of the differential. You may find some little tabs back there spotwelded onto the floor pan. As I recall, the twin running board antenna wire is in the neighborhood of fifty feet long. There was a connection at about the center of the crossover, so each segment was about 25' long. I used black rubber spark plug wire, since that's what the original stuff looked like. There were remnants of the twin RB antenna still on my car when I got it, and the '36 (parts?) manual had good pictures. The wire was rigged with small S-hooks and rubber rings. I used small O-rings.

Well, you know the ideal length for an AM antenna is yards and yards long. The radio wizards hadn't yet figured out how to make the radio circuitry "think" it was looking at a long wire when the antenna was only 3 feet long. That came just a few years later. So meanwhile, the longer the better for reception.

I drove my '36 about 300 miles a year for two years before parting with it, and never had a problem with the antenna. The O-rings had just enouigh stretch to keep the antenna wire pretty taught. Note that you can barely see it in this photo.