According to www.oldgmctrucks.com it looks like a 1/2 ton, 116" wheelbase, 228 engine, came off the Oakland, CA assy line in 1951.
This is the best website I've found for GMCs of this era. The webmaster frequents this board and he will answer your questions by E-mail.
The only problem I have with the site is knowing when to leave it.

"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.