Lets run with the story for a bit. If it was used in Vietnam by the Australian SAS would/should it have been marked "ASAS" or "Australian SAS", assuming that they wished to mark their weapon?
When the "Australian SAS" is being referred to in the UK it is normally referred to as the "Australian SAS" so as not to confuse it with the "UK SAS". But in Australia, is the "Australian SAS" referred to simply as the "SAS" or is it still referred to as the "Australian SAS"?