its a right side OPTIONAL tail light arm and where the factory license plate light bracket would attach its filled with a steel cap that just fits into that gap, is held there with the steel rivets.
value, have seen these priced all over the place from $20 up to $400.
I have 3 1935 Chevrolets and they all have this exact same right side "optional" arm.
There is a repo aluminum arm about $75 out there without the separate cap
mike lynch