It is a replacement coupe shell body in white, most likely a HQ but also could be a HJ. There were some of these still for sale within GMH in 1980. You could buy a HJ coupe shell p/n 9932645 for $600 as a runout to clear such stuff. They normally put the part number on them as the body number but I have seen them like this before at some stage. See if you can find out what it is on and if the shell has a number on the firewall above the heater box.