I thought may be something to do with what level of CASA approval they have ?
Nothing quite so inscrutable as approval levels ... The difference is just a consequence of changed administration.
In the olden days of Pontius the Pilot, the regulator was DCA (the original regulator) and fairly decentralised. There were several Regional Offices and they exercised a fairly autonomous oversight of their geographical bit of the Australian Industry. Head Office, then in Melbourne town, nominally looked after policy and other high level stuff while spending a lot of time trying to keep the Regions in some sort of consistent order, but that's quite another story.
Up until the mid-70s, weight control approval was retained by DCA except for airline work which, so far as I am aware, was delegated to the airline tech service organisations. For GA (even extending to the DCA Flying Unit for which I worked in the early 70s and was, to all intents and purposes, every bit an airline in capability) weight and balance stuff was prepared by Industry folks and then submitted to DCA for approval by airworthiness engineering folks.
Around the early- to mid-70s stuff started being delegated to Industry in earnest and this included weight control work. The present weight control system (ANO, and subsequently CAO 100.28) was introduced in July 1976 and provided for the issue of weight control authorities to Industry folks. As the Regional Offices administered these authorisations, the original authority IDs were aligned to the Regions. So, the early folks who picked up tickets were identified by an alphanumeric ID which comprised the Region identifier and a consecutive number to indicate the order in which authorities were issued.
Some of the early authorities, which I can bring to mind, include
Queensland Region
Bill Whitney AQ2
Jim Liddell AQ6
NSW Region
Bruce Clissold AN9
South Australia Region
Peter Denholm AS2
Victoria/Tasmania Region
John Heddles (me) AV1 - while of no particular importance, mine was the first issued at start of business on day one
Bing Molyneux AV29
Some considerable time later, the Regions were subsumed to some extent into Central Office and the administrative control of weight control work moved from the airworthiness engineering group to the maintenance surveyor group. As part of this change, the weight control authorities moved to a single control system with the authorities identified by "A" and the ARN, as I gather.
Those of us who were in the olde phartes group retained our old Regional indentifiers and you will continue to see us represented in aircraft paperwork until we all die off. So, for instance with respect to the previous listing, Bill is still active, Jim I lost contact with years ago, Bruce is still active but in semi-retirement, Peter I lost contact with years ago (although I see that he is on the Kangaroo Island council and, no doubt, still going strong), I'm still in the game while BIng (lovely chap that he was) died some years ago.
It follows that we old chaps have ARNs which are very much smaller numbers than what one sees being issued these days. Mine, for example, is 6784, dating back to the mid-60s when I started flying.
