George,
Beaky has you on the right track to set it via the dips, it's function 5 (Maximum credits, set to a value of 0) on test number 4. Having fiddled with my WC I can see why WMS abandoned this method of changing settings
Keep in mind if you do set it to free play via the dips then the knocker seems to stop working - so no knock when you start a game, win a match or achieve a high score. I love this sound as a feature on pins.
You can also mount an extra switch on the back of the start button which is wired to the coin microswitch. Pushing start adds a credit and starts a game, knocker still functions.
I'm still digging out those switches for you, will try get them in the post on the weekend.
