Here's the run-down on the model numbers:
M923/A1/A2, Cargo Truck, WO/W (Dropside)
M925/A1/A2, Cargo Truck, W/W (Dropside)
M927/A1/A2, Cargo Truck, WO/W (XLWB)
M928/A1/A2, Cargo Truck, W/W (XLWB)
M929/A1/A2, Dump Truck, WO/W
M930/A1/A2, Dump Truck, W/W
M931/A1/A2, Tractor Truck, WO/W...