It’s the original EZPass.
Truly a great, rare, and unique piece of Virginia history. Below are some excerpts from the Wikipedia article “Boulevard Bridge” that mention this type of license plate add-on
At 2,030 feet (620 m) long, the Boulevard Bridge was completed in 1925. It was privately owned and financed by the Boulevard Bridge Corporation for the purpose of providing access to the new Westover Hills neighborhood in South Richmond, where one of the selling points of the homes was free bridge access.
During the years of ownership by the Boulevard Bridge Corporation, Westover Hills residents were given free access across the bridge  by the use of a special license plate that was attached below the state plates on their cars. These plates had to be applied for and were also sold to the general public. They were changed each year, and in the last years before Richmond Metropolitan Authority (RMA) control, windshield stickers were used.
As of this morning the bid was at $19 but be warned I’m probably going after it.