Used to get this on the METAR for YBAS and a couple other ports in the Centre up to two or three years ago.
No operational significance apparently. It indicates to the BoM user that there has been a change of equipment at the auto met station. New equipment or software upgrade is monitored to see if it is producing different outputs to the previous.
Give the BoM officer nearest that station a call, numbers are given in JEPP ATC pages AU-604 and AU-605.
Mentioned to the BoM that a NOTAM explaining the abbreviation would have been nice -- save a few hundred pilots thumbing through MET Definitions.