Mike, Your truck is a "Rare" 1 1/4 ton truck. Not many of those were made. You'll notice it has the same small truck grille, as the 1/2 ton, through 1 ton trucks, with a extra piece attached to the splash panel below the grille, to fill the space between the grille, and the channel bumper. Everything else looks like a 300 series, 1 1/2 ton. The 300 series, and larger trucks, have a larger grille. Here's a 280 series 1 1/4 ton, next to a 300 series 1 1/2 ton. A friend of mine owns these two trucks.
Spanky Hardy Collector Of Fine Old G.M. COE Trucks & Antique Holmes Wreckers
