Hi,
The 5 windows trucks were sold as deluxe cabs. The deluxe cab had 5 windows, maybe a radio, maybe a spare tire mounted on the side of the truck, maybe a fulton sunvisor. The 5 window cabs are getting hardder and harrder to find. Have fun restoring your truck. Here are some places you can get parts.
LMC Truck
Classic Parts
Classic Industries
CARQUEST
NAPA
Chevytruck daddy