@lauren @kevin @mmasnick
Exactly, only one extension could be used at a time.

Admittedly, I was 10 when I first encountered this and it was in locations controlled by the US Military (offices and off-base housing) in West Berlin, so I don't know if this was a standard thing or if I misunderstood its purpose, etc. I never saw it anywhere else after that or during the 12 years I spent in Southern Germany in the 1980s-90s.