The door lock thing was required by law in some areas until the '60s - you weren't supposed to exit a parallel-parked vehicle except through the curb side. Therefore, you locked the driver's door from the inside, and climbed out through the passenger side.
If your '49 still has the drop-down handle doors, you're mostly stuck on the door lock. The only way I know of to get a driver's side key lock is to switch to doors with the push-button handles and vent windows (1952-1955.1). I think there are kits to eliminate the vent windows on those doors. If you have the push-button doors handles, you can have another lock button keyed to match your original lock button, and have one on each side.
