That's the optional V54 roof top carrier for the 80-82 big back window cars.  It was not a Corvette option for 78 and 79, but would fit.  Looks like a good one at a good price.

It is also missing the Bag that came with it to store all the Parts when not in use. Some 80 to 82's came with it from the factory but it was still sold over the Parts Counter. Most People Cringed at Drilling 1/2" or bigger Holes in the Rear Deck to Mount it. The Screws that go in the Rear Deck are Plastic and Crack Easy. No seperate Parts were ever Sold. When the unit is mounted and used it Giggles a bit and the Rubber Pads Mar the Paint and You need to Buff it out a bit when Your done. Clear coat Paint might be much better. This unit was made in Canada.