A typical number would be something like 4166-1153. The four numbers before the hyphen are comprised of the model year (40, 41, 42) and a 2 digit body code drawn from the following list:
The year code indicates the model year designated by the manufacturer and not necessarily the calendar year when the vehicle was built. Thus, "40" meaning 1940 indicates a VC/VF model; "41" representing 1941 indicates a 1/2-ton WC model and "42" for 1942 refers to 3/4 & 1-1/2 ton WC models.
The numbers after the hyphen indicate the production number of the vehicle body type, so in the above example, it would refer to a 1/2-ton Weapons Carrier with an express body fitted with transverse rear seats - the 1,153rd built.
A 4 or 5 digit number can also be found stamped into the engine compartment side of the firewall on 3/4-ton command cars - next to the steering column hole. This appears to be a Budd serial number used only on command car bodies - it may not necessarily match the body tag number as the same bodies were used to produce both command cars (type 19) and radio command cars (type 29).