Gentlemen thanks for your help so far.
I have found two more pictures with this device.
Both are 3 ton RAF Ford trucks, so maybe it is a RAF specific thing?
Strange also is that I have found this only on Ford trucks??
It may not be appropriate to put on an 15cwt RAAF ford truck
I agree it is a strange place for a AA mount, but I have no idear what els it could be.
All the best
Leon