Thanks everybody for your input, As said earlier I have seen a few pics of this on a few other GMCs but yes seems to be very rare.
It is made very well...all the welds and fab are top notch certainly not out of the realm of factory manufacture. I'm now thinking maybe Western Auto or another catalog dealer of the day could have sold these. And I agree it is certainly a conversation piece. It would be a perfect match up for pulling a classic 50s teardrop trailer or something else along those lines but that's not in the cards of this old farm truck. So off it will stay. Anybody interested in it let me know.