I believe the rear louvers were an option on the 77 cruising wagon. Not sure if an aftermarket company made some. All 77-80 louvers should be the same and should fit all years of wagons as the rear glass is the same on all wagons.
My 78 CW looked like it had a set at one time but they were not on the car when I bought it /owned it. I have found a few sets since I sold my CW. They are out there, earlier this year there were 2 different sets on CL locally. Good luck