The easiest way to find the differences is to download the tech manuals, look in the parts manuals, find the UOC (Used on Code) at the beinging of the manual and then see what the different part numbers are. If there is no UOC then the parts are the same.
I do this all the time with the CUCV parts. The majority of the parts are the same but sometimes there is a difference between models.
http://www.steelsoldiers.com/showthread.php?79228-TM-s-for-the-939-series-trucks
http://www.steelsoldiers.com/upload/939/TM9-2320-272-24P-1.pdf
5. Special Information.
a. Usable on Code. The Usable on Code appears in the lower left corner of the DESCRIPTION column heading.
Usable on Codes are shown as UOC: . . . . . . .in the DESCRIPTION column (justified left) on the first line of applicable
item description/nomenclature. Uncoded items are applicable to all models and title description will indicate all M939,
M939A1 or M939A2 models. Identification of the Usable on Codes used in the RPSTL are:
Code Used On Code Used On Code Used On
V15 M923 WO/W
DAW M923A1 WO/W
ZAA M923A2 WO/W
V13 M924 WO/W
DAA M924A1 WO/W
V14 M925 W/W
DAX M925A1 W/W
ZAB M925A2 W/W
V12 M926 W/W
DAB M926A1 W/W
V17 M927 WO/W
DAC M927A1 WO/W
ZAC M927A2 WO/W
V16 M928 W/W
DAD M928A1 W/W
ZAD M928A2 W/W
V20 M929 WO/W
DAE M929A1 WO/W
ZAE M929A2 W/W
V19 M930 W/W
DAF M930A1 WA/
ZAF M930A2 W/W
V22 M931 WO/W
ZAG M931A2 W/W
V21 M932 W/W
DAG M932A1 WO/W
DAH M932A1 W/W
ZAH M932A2 WAW
V24 M934 WO/W
DAJ M934A1 WO/W
ZAJ M934A2 WO/W
V25 M935 WO/W
DAK M935A1 WO/W
ZAK M935A2 WO/W
V18 M936 W/W
DAL M936A1 WO/W
ZAL M936A2 W/W
V39 M945 W/W
Looking for your M923A2 it shows a UOC of "ZAA". When looking for specific parts see if any of the individual items has a Code. If not then the part will fit all the variants of the truck.
The Fuel cap is listed in Figure 82 on page 82-1, item #2. Fuel cap is part number 12356774. No UOC is listed so the cap will fit all the M939 series trucks.
Now looking at the tank iteself, item #1, you will see 4 different part numbers. ZAA is listed under the first number. The tank is shared along with the other associated UOC codes.
