As time moved on (like sand through an hourglass!) so did Infantry markings and terminology. The S still represents Support Company but later on what was previously known as an Anti Tank (or Armour) Platoon became known as a Weapons Platoon instead. Therefore, vehicles that were, in the earlier days of the Vietnam conflict numbered SA 3,4,5and 6 were to become SW 3,4,5,and 6.
This was not a rare thing to happen as during the reorganisation of the army in the mid 70s some Companies became Squadrons and Platoons became troops etc.etc., similarly in earlier days wirelesses became radios.
Same animal, different name.
Regards
Glen