I installed one in the back window of our '31 Chrysler sedan and it works well and is very bright even in daylight. On our '31 A roadster, there was no window to install it on so I elected to use replacement lenses and LED lights from Snyders in both tail lights. They also are very bright and look completely stock.
If I had another Model A coupe or sedan, I would install the replacement lenses and LED's because they are brighter than the 3rd tail lights in the rear window. IMO, they are also better because modern motorists following you are accustomed to and expect braking and directional signals to come from tail lights and not from a 12" light in the rear window.
I have both types and feel having two rear tail lights with upgraded LED's is better and safer than the 12" horizontal bar in the rear window. However, since your "mistress" has been gifted the 3rd tail light, by all means install it. It will be far better than the stock A lights and you will have a safer car to drive. Better yet, install both systems for the ultimate visibility and safety.
I installed the 3rd brake light on the Chrysler some time ago along with directionals which were already mounted on the car and had some difficulty making them work. Had to install resistors in each tail light wire to get them to work properly. As I recall, the instructions at that time did not mention the need for resistors when using the light with 6 V directional signals. I bought the light direct from the manufacturer and had a few phone calls with the owner regarding required wiring and resistors before we could get them working. Hope the current directions are more up to date and you hit no roadblocks in your installation. After you are done with the installation, you will be pleased with the results.
Glen