Blue wrote:
Taking a closer look a at those lap belts, they don't look like original Mopar parts to me, I think someone has replaced the originals for some reason.
I think you've cracked it, Blue.
I've just been having a nose on ebay US (I guess that's going to be my night time entertainment from now on!) and I've found a set of belts advertised for a '72 duster with totally different lap belts.
http://www.ebay.com/itm/mopar-a-body-72 ... 2164701730
These clearly show a little mushroom on the male half of the clasp as you described which would match up with what I have bolted into the roof of the car.
Additionally, the female front clasps in our car are a different style to those fitted in the rear. However, the female clasps in the advert above match the style of the units fitted to the rear of our demon exactly?
So, as you say, I guess the belts in the roof are right and the lap belts need to be swapped out.
At least I know it's not me being stupid now!

-Jon.