Hi Andy

In the 1955 CHEVROLET TRUCK 2nd Series OPERATOR\'S MANUAL on PAGE 11 is an explaination of the door locks.

As some of the guys mentioned, the keyed lock is on the passenger side door and to lock the driver's door, just push the inside door handle forward to lock.

Now you are probably wondering why the outside door lock is just on the passenger door. Could be several reasons such as when you pull to the curb, you lock the driver's side door and slide out the seat on the passenger side thus not getting run over by the town drunk. Another is being the gentleman you are, you will exit the driver's side and your lady will lock the door for you and you will go around to the passenger side and open the door and assist your lady from the truck.

Hope this helps!

Cheers!