Help - What Are These For?? Old Canopy Option?

[my71gmc]

I bought my 71 GMC Jimmy about 13 years ago from its 2nd owner. When it arrived from California, it had these 'tubes' mounted in the rear of it. For years I have been trying to figure out what they are for, but couldn't so have just had them in the shop... I finally got ahold of the guy I bought it from and he said that when he bought it from the original owner, that guy told him they were used for a 'canopy type option'. There was no canopy with it, just these tubes.

Any ideas???

Thanks!