Well I have had replies from Bay Area Amusements and Marco Specialities advising that an alternative armature and extension is an 02-4668. This is armature that is used on the multi-ball popper. They both advised that the plastic extension needs to be cut down and the ball popper cap attached.

Unfortunately they don't have details of the length of the extension. Can someone with a WH20 possibly measure the length of the plastic extension from the  join to the base of the ball popper cap?

Cheers,

Andy