As far as I know, most of the sliding window export models that found their way to the United States did not have center brake lights. Most of the Isettas exported to the US had a black opaque plastic insert manufactured by Hella that provided illumination only for the license plate.
As you can see, my tag light lens is in pieces. One side of the lens was already broken when I removed the housing. After I had carefully ground the head off the rivet on the other side, I was pushing it out and broke the other side. WARNING - these lenses are VERY brittle and easily break.
The plastic of the US tag light insert looks to have been crudely notched by hand to accept a different type of bulb and socket than what was originally intended. I guess either Hella or BMW did not want to spend the money making a new mold for the US export models.
I wanted to have a center brake light on my Isetta so I picked up a new reproduction brake/tag light from Isettas "R" Us. Now I need to call Werner and see if he has a replacement for the tag lens I broke.