In the hopes it may shed a little more light on the question, here's some info I found looking through both the GM Heritage Data PDFs, a couple different Chevy Master Parts Books and the associated Chevy Truck Shop manuals covering the 1946 trucks.
1 - There were 2 different Series "Designs" according to the Parts and Shop Manual Model Identification tables (GM Heritage PDF only lists the 2nd Series identifiers for unknown reasons). The 1946 "First Design" used the letters "OS" to indicate a 160" wheelbase Utility 1-1/2 ton truck in the serial number sequence. The same size/model truck designated as a 1946 "2nd Series" is identified by the letters "PK" in the serial number sequence. In both cases these trucks were assigned Model# 4400.
2 - Both series 1946 1-1/2 ton trucks could be ordered with a 2 ton heavy duty front axle (same as used on COEs and School Bus Chassis). Those heavy duty axles were identified as the front wheel hub cap is attached to the hub with 6 1/4"-20x7/8" hex head bolts on a 3-7/8" diameter bolt circle. As far as I can tell, the addition of the heavy duty front axle is the basis for the increased GVW rating of 14,000 vs the original GVW rating of 13,000.
3 - The "S" identifier that is mentioned above by TwinStickOS only applies to the 6100 and 6400 Series 2 Ton Trucks. Those have the initial Serial Number ID Letters "PV" and "PW", so that special designator doesn't apply to the "OS" ID trucks.
Decoding MDReed's truck serial#: 20 = Van Nuys CA Plant / OS = 160" WB 4400 Utility 1-1/2 ton truck / 10 = October (build month) / 1314 = 314th 1946 1st Series truck built at the Van Nuys plant.
Decoding TwinstickOS's truck serial#: 20 = Van Nuys CA Plant / OS = 160" WB 4400 Utility 1-1/2 ton truck / 10 = October (build month) / 1724 = 724th 1946 1st Series truck built at the Van Nuys Plant.
Here are additional links to info resources on your trucks:
1946 Chevrolet Truck Data PDF1929-54 Chevy Master Parts Book - Serial Number Data (1946 Truck info is in lower left of page)
1946 Chevrolet Construction Trucks PDF (Includes lots more specialty equipment data not included in TwinstickOS's linked brochure)
Hope you find this information helpful.
