In Vietnam back in the 60s, 1 Fd Regt, was represented by 42 on a red/blue sign.
4 Fd Regt wore 43.
12 Fd Regt wore the unit number 44 on their vehicles.
The third tac sign that was common on a lot of RAA vehicles usually denotes the battery. A Regiment consisted usually of 4 Batteries each comprising 6 guns. As far as I can glean, each vehicle wore the Tac sign of the parent Fd Regiment but the battery was represented by the position of the red quarter on the third Tac sign that was pretty well unique to Artillery. This coloured quarter floated around from top left in a clockwise direction depicting the various batteries in that regiment beginning with the HQ battery- top left and so on. This can be a bit hard to follow as often batteries floated around from Regiment to Regiment at times as requirements and changes were applied. Sometimes a letter was used instead, depending on the unit. These "third Tac signs" were really for internal use more than anything else.
During the Vietnam conflict the original 1 RAR detachment that went over in 65 was supported by the Kiwi 161 Battery (May 65) and their Tac sign was 13 displayed on the red/blue Artillery colours background. They were later supported by 105 battery RAA (Sept 65), and later in mid 66 when the Task force commitment occurred at Nui Dat 1 Fd Regt was deployed in full, comprising HQ battery, 101, 105 with the Kiwi 161 Battery filling the fourth Battery space.
Of course, your vehicle being a S3, none of this really is that significant, but may be of interest to some with earlier vehicles
In later years the numbers still represent the same units but the coloured backgrounds have disappeared and a Tac sign now is usually black numbers on an olive drab background, making it a bit harder for outsiders to recognise as well as a bit harder to see.
Regards
Glen
