Funny - I just stumbled across such a "double click socket" in the middle of Europe! A friend of mine has an imported US floor lamp and mentioned the weird switch today... just ditched everything but the shade today though (the socket had a bad internal short arcing to the bottom part of the bulb screw shell (next to the black stuff that separates center and shell)... and since the thread on the lamp is US it prove impossible to replace the shot socket with a European one which has a metric thread...
