I have a hunch it depended on the plant on the GMC's. I have New Design parts trucks from both the Oakland and St. Louis plants. They have clutch heads. Both these plants also built Chevy trucks. The Pontiac plant was all GMC. I suspect that's where the ones with Phillips were made.

"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.