Some of the military GMC were 24V. The dropping resistor reo_guy saw listed may be 24V to 12V. I'm almost certain there were no 12V civilian trucks, 2-ton and below, before the V8s as GMC panel has listed. I think some of the larger GMCs may have been 12V before but I don't have documentation on what models.

"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.