Right at the start the film is dated 1970, 3 RAR training at Woodside prior to a February 71 detachment.
The instructor (HKFF) is pretty safe standing where he is as no ammo is loaded at all during the exercise. The badge on his slouch hat looks like an infantry badge to me and as said earlier that dates the photograph to pre 72. The uniforms they are wearing also date them to around the 70's mark as the trousers are the more modern American cut rather than the baggy ones that were issued in earlier years, and the jackets are the "pixie" jackets, also taken from the American model.
These blokes are not on their first training exercise either as it is pretty obvious as the barrel traverses the loader and crewie both move around in relationship with the barrels movement. Note the driver unlocks the barrel clamp.
I don't know what type of screens you are watching this film on but on mine I can see that the vehicle is wearing a 57 tac sign on the bottom right corner of the NS guard and the squarish sign on the grille looks like a SA4 but a bit hard to read.
It is a S2a and fitted with a brush guard. ARN is a bit harder.
The question is whether this actual vehicle went to Vietnam with 3 RAR or did they simply take over the existing vehicles in country.
By 1970-71 the writing was already on the wall that we were pulling out in 1972 so I can't really see that the vehicles would have been replaced for such a short period of time unless of course the existing ones were well and truly BER'ed.
I am under the belief that Onslow returned to country in 1972 with 3 RAR and according to photographs, know that he was in country prior to their arrival, serving with 6 RAR on their second tour and also with 9 RAR, so that is at least one that 3RAR took over from "in country" stocks.
Regards
Glen