Side impact bars

While I was looking for engine bay photos, I came across the x-ray pic below by Bret M. Duff (Bret, hope it's ok I repost it):

Was puzzled by the very prominent horizontal bar going through the middle of the door, and started wondering if Porsche was fitting side impact bars long before they became standard on virtually all cars.
A search in the forum indicated that it is indeed a side impact bar, apparently required by law in the U.S, but optional in ROW. Someone suggested it became standard in later model years. Anyone have more info on this ?
Cheers,
Erling



