So I've just clicked on 'view an explanation of the service records' link and it shows this statement:
Quote:
In selecting the information to be displayed on the website, the Department was aware that some Defence Force members had multiple periods of service. The Nominal Roll aims to show a separate entry for each continuous period of service within the one armed service so that breaks in service are clear. However, particularly for Army veterans, the way the individual's service dossier was managed during, and after, the War made this a difficult task. In some individual cases, each period of service was filed on its own dossier with its unique service number. In other cases, the periods of service had been amalgamated into one service dossier and stored under the one service number. This service number is usually the number for the latter period of service. The Department will, on a case by case basis, review an individual's dossier/s and amend the details displayed on the Nominal Roll.
The service number under which the paper service record is stored is displayed first. Any other known service numbers relevant to this period of service are displayed in brackets.
So we were on the right track! A31741 was his service number for the latter part of his service, and 5665 was for the early part - being WW2 service.
It doesn't explain the typo at all, but if it is just a typo then nothing can explain it - and I think this is the case.