You may not have the original stock wheels. On a 56 car the wheels have small nubs on the inside portion of the wheel. The nubs hold the full size cap on. I believe this is the case with your truck also as they use the same cap. The 57-58 Cameo used clips to hold the beauty ring on but they are completly different. Your stock wheels could be from a later model and the aftermarket wheels also won't have the nubs.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine