I've got a couple of WMS machines of this vintage and that is how they seem to work - in freeplay no knocker. I've never noticed the credit count not displaying, but it seems reasonable as don't you set the max credits to 0 to put it into free play. I've always meant to check if the games played etc statistics continue to work in freeplay.
Maybe this reason is why some nong bored a hole and mounted a bright green "coin" button on my Firepower - looks really great!
BTW the lack of the knocker sound doesn't worry as much on Firepower as Gorgar as I get many fewer free games on Firepower. On Gorgar you seem to get them much more easily.
I did read in someone's website or guide of the idea of mounting an extra switch contact set on the play button and wiring this extra switch to the coin micro, set it so this additional switch activates slightly before the play button. Now every time you push play you get the sequence add a coin and start a game and pin can be left with more normal replay settings.
