why the clear dimple on only one tail light lens

[Lindsay McConnell]

Originally, the lens with the clear dot was mounted on the left side of the car. This dot did have a mirrored backing to it that in the evening, and especially at night, would reflect back the headlight of an on coming car back to it's driver so that it hopefully would keep the driver from hitting the parked car.
In 1941, there was no outside lock on the left side too. This kept someone off of that side of the car fumbling for keys to enter. All of these were simply safety issues. This was changed after the war.

[Andy Wylie]

It's intended to be a reflector but the gun metal colored reflective material seems to have fallen off most of them.

[1953 plymouth]

i have a 41 ply coupe and a 1953 belvedere and both cars have a clear area in the lens on the right tail light only. does anyone know the purpose for this?