Neil, I believe it's an impulse because the switch mechanism centers before you can pull out the key. Also, I have a bad driver side alarm switch and I use the same key motion to set and unset the alarm - I unlock the driver's door, open the door, twist the key to lock quickly so the alarm doesn't blare. Also confirmed by the W126 wiring diagram.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD