I had a 1964 Mercedes Benz 190 that had that feature - drove me nuts..........
I took the lock apart and found that there was a cam added to the mechanism
fixed to a shaft with a roll pin.  Took it off and the door would lock with the
button down.
Looks like it was added for specific markets.

Randy Bergum
1990 Vanagon Carat

Michael Snow wrote:

> Have you ever locked yourself out of a car?  Breaking into your own vehicle
> or hiring a locksmith is far more expensive than replacing door locks every
> 20+ years or so.  I think this is one of the more intelligent features of
> the Vanagon.  At least some Subaru cars use the same design, and they also
> reduce the number of dead battery surprises by running the headlight relay
> through the ignition switch so the lights cannot be inadvertently left on.
> Smart...
>
> Mike Snow
>
> <snip>
> >Has anyone ever modified thier Drivers Side Door Lock so a person can lock
> >it by pushing the lock button down, instead of having to use the key?
>
> I want to know too; it's a real PAIN having to use the ket to LOCK the
> door, and wears the lock out twice as fast.
>
> <snip>