I've seen a few of these vehicles, but they are quite uncommon. There's no truth to the story about being short of wagon bodies, although the three-door is panel-van based. There was at least one body-builder / coachmaker who specialised in these conversions. They were used as ambulances within the military and similar uses. Ken McLean (a regular here) restored one of these about 8 years ago, and knows more about them than probably anyone.
Hope that helps, RET
PS - I would like to see the photo:
ret@nsw.fefcholden.org.au