TB 43-0209 - Color, Marking and Camouflage Painting of Military Vehicles, Construction Equipment and Materials Handling Equipment dated 31 October 1990, Section III – Identification Markings, paragraph 9 – Unit Identification, sub-paragraph n – Typical Examples, has the following example for Fifth Army shown in sub-sub-paragraph (14):
5A-531T TRK-7 Translation: Fifth Army, 531[SUP]st[/SUP] Transportation Company, 7[SUP]th[/SUP] Vehicle
The bumper number in the photo shown above, 5A-816-O X-93, is most likely for vehicle number 93 of the 816[SUP]th[/SUP] Ordnance Company assigned to the 5[SUP]th[/SUP] Army. Internet searches turn up references to the 816th Ordnance Company. TB 43-0209 says the designator for Ordnance is the letter “O” preceded by a dash. The dash is to separate it from the numbers so it won’t be miss-read as a zero. In the photo, there seems to be more space between the 6 and the “O”, just no dash. In Separate or numbered companies, “X” was often used in position 3 as the unit designation was already located in position 2, different than letter Companies organic to a Battalion in which the letter designation, A, B, C, D or HHC would replace the X.
TB 43-0209 says the designator for Military Police is the letter “P”. So the bumper number of a vehicle assigned to a Military Police Company under 5[SUP]th[/SUP] Army would be: 5A-123P X-25 for vehicle 25 of the 123[SUP]rd[/SUP] MP Company assigned to the 5[SUP]th[/SUP] Army.
X-1 would be the Company Commander. X-2 through X-9 would be Company Headquarters, Supply and Operations Section vehicles. X-10 through X-19 would be 1[SUP]st[/SUP] Platoon with X-10 being the Platoon Leaders vehicle, X-20 through X29 would be 2[SUP]nd[/SUP] Platoon, ditto 3[SUP]rd[/SUP] Platoon and 4[SUP]th[/SUP] Platoon. This numbering system seemed to be used by many Combat Support and Combat Service Support units, different than that used by Combat units where the Commander is always in vehicle 6, i.e. HQ-6, HHC-6, A-6, B-6, etc. and HQ-1 being the S-1, HQ-2 the S-2, HQ-3 the S-3, HQ-4 the S-4, HQ-5 the XO and HQ-7 the Sergeant Major or First Sergeant.
This Technical Bulletin can be downloaded from:
TB 43-0209 The information about bumper markings is on pages 15-17.
The 29 October 1976 version of this Technical Bulletin is in the Steel Soldiers Miscellaneous TM section of the TM Forum.
Just in case I inserted the above link wrong:
http://www.dmkf.dk/Filer_manualer/Camo/TB 43-0209 color marking.pdf