Mercedes-Benz 300 SL Operator's Manual page 30

Note:
The central locking system can only be
engaged at the passenger door if the
key is removed from the steering lock
or is inserted in the steering lock
without having been turned.
If a door is not properly closed, unlock
the car, open and reclose the door, and
lock the car again.
Locking and Unlocking at
Trunk Lock
1 Neutral position
2 Unlocking
3 Locking (detent)
4 Separate locking of trunk
Locking
Turn master key to position 3, turn
back to position 1 and remove.
Unlocking
Turn master key to position 2, turn
back to position 1 and remove.
Separate Locking and Unlocking
of Trunk
If the trunk is to remain locked (e.g.
when leaving the car at a parking
garage or workshop), lock the trunk
separately. Turn master key to position
4 and remove in this position. Leave
only the valet key with the vehicle.
The trunk (and fuel filler flap) remain
locked - even if the central locking
system is engaged at a door from
outside or inside the car (theft
deterrence).
To unlock turn master key to position
2, then back to position 1 and remove.
30