What version game ROM do you have? I am confused...

Suffice it to say that my guess is that the current ROM does not have the knocker functionality. Anyone else like to comment on whether that is a possibility?

Of course, we could be barking up the wrong tree on this one.

In the interim, is there any chance of you tracing the violet and black wires from the coil back to the driver board, and checking continuity between the coil end and driver end of the wires (with the machine turned off)? (Assuming they are violet and black - that's what the manual said.)

Adjust multimeter to 200 ohms setting.
Place one probe end on the coil end where the violet wire attaches.
Place the other probe end on the driver board where the violet wire attaches.
You should see a zero reading.

Repeat for black wire.

ROM version is LC-4, I have ordered ROM revision 9.4H from Pinbits so perhaps that will solve the problem.

I wasn't sure if I needed to switch the machine off at the mains or not, so I left it on at the mains but turned it off at the switch on the machine. Using the MM (set to ohms at 200), I put a probe on one of the lugs on the coil and the other probe on the wire at J130 on the driver board. The reading on the MM was 0.6. I then put a probe on the other lug on the coil and the other probe on the wire at J107 on the driver board. Again the reading on the MM was 0.6. Given the readings were not zero, does that indicate a problem? 

Matthew