
A long time ago on another forum this photograph of a particular vehicle was posted and created a bit of interest as to its ownership at the time of the photograph (1965).
Somewhere in REMLR.com there is a page that offers information re door markings and how to recognise who or what
certain markings represent. The Diamond, Triangle, square, circle and two rectangles (vertical and horizontal ) represent, in order, HQ, 1 or A, 2 or B, 3 or C, 4 or D, 5 or E, depending on the era. These can either represent Batteries, Companies or Squadrons, depending on the Corps. (That is represented by the colours of the Tac sign).
The following concerns the "pentropic" set-up under which most units headed off to Vietnam, and generally an infantry Battalion in those days used letters to define its Companies, A Coy, B Coy etc., and numerals to define Platoons and sections.
A Normal Infantry Battalion consisted of 5 Companies, under a Head Quarters group.
HQ consisted of Admin, Sigs and Q mainly.
There were generally 3 or4 Rifle Companies, usually labelled alphabetically, A, B, C, and D.
These Companies themselves consisted of a HQ element, Sigs element and 4 Rifle Platoons.
Then there was Support Company.
Support consisted of a Pioneer Platoon, Mortar Platoon, Sigs Platoon, and Anti Tank Platoon.
This set up may have differed from Battalion to Battalion as some had different primary roles and would have been set up slightly differently.
The OC of each of these sub sections would have been entitled to a vehicle and his vehicle would have been allocated the number 1,
The 2ic would have had #2, and Admin and Q would have probably had #3 and #4.
So in the case of seeing the above short base Land Rover rumbling towards oneself, the first thing to look at was the colour of the Tac sign; Red with white numbers- Infantry. 55 represents 1 RAR. The small sign on the grille tells us 'S' for Support Company, 'P' tells us Pioneers and the number '1' tells us it is the OCs vehicle.
The one behind it is the OC of Mortar Platoon in Support Company.
Had there been a gunbuggy in the picture it would have been listed as S (Support) A (Anti Tank) 4, 5, 6 etc., depending on which vehicle it was. Anti tank would have had extra vehicles allotted as most of its weapons were vehicle mounted.
What this means is that there were at the time of the conflict, nine Battalions raised and each one would have used a similar numbering technique, so if we see a gunbuggy numbered SA6 we don't really know which one it is as each Battalion had a vehicle marked SA6. The Tac sign becomes the important issue for recognition here.
When we are talking about a Transport Platoon, a similar system operates, the Tac sign tells the parent Company, the shape on the door tells the Platoon and the numbers in that shape tell the section and the vehicle number. Therefore a truck with a Triangle on the door and the numbers 53 on the door belongs to 1 Coy RAASC (Tac sign) 1 Platoon (Triangle) 5 Section and it is truck no. 3. (It can't be number 53 as a Tpt Platoon only had 33 vehicles!)

There may be some old Infantry Tpt NCOs out there who may elaborate on the above generalisation, or even some Artillery bods who can throw some light on the interesting system the Drop shorts used.
I Hope this makes some sense and is of assistance to some.
Regards
Glen