I have a friend who was a Qantas outstations engineer on these aircraft but he is on holidays right now. However while I wait his return I am wondering is it an Australian DME antenna.
Looking at all the photos that I have of Connies of that era I agree its exclusive to Qantas 749s so it has to be a system that was exclusively Australian. Australia at that time had invented and operated its own DME system. So maybe that's it.
I'll update when I find my friend
Wunwing