Bingo. That set a light on in my head. There were no long bed Chevrolet half tons in those years. That's why it has to be a GMC assembly.

However, according to Hollander, the wheel cylinder is the same, GMC or Chevrolet, half ton to 2 ton, 1937-50.

"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.