I have deduced, from previous posts, that the practice started in 1955 but wasn't done at all plants until 1956. I have also deduced it wasn't done to GMCs, even though some GMCs were produced at Chevy plants. I haven't determined if GMCs and Chevies came down the same assembly line.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.