An extract from the Australian War Memorial’s website mentions the AC III tanks and of note is this particular passage…
The Memorial's ACIII variant is an upgunned variation of the ACI, and utilises a modified 25pdr field gun for increased firepower. This necessitated considerable design modification, mainly in the provision of a larger turret and turret ring and deletion of the hull machine gunner's position. This Mark also included the redesigned and compact engine installation featuring a common crankcase; allowing room for extra fuel tanks.
The ACIII was delivered to the Army on 12 June 1943. The ACIII with Perrier Cadillac engine was given the name Thunderbolt. The ARN is 8066. This was the only AC III to be built although another may have been started, however with the cancellation of the AC programme in August 1943 no more were completed.
This seems clear enough that the name ‘Thunderbolt‘ was the name given to the individual vehicle (a standard trait in all Commonwealth militaries) and not a generic name for the proposed production tank - in a similar manner British Sherman (series name) tanks were all given individual names by their units/crews. As
@Bougnas points out the AC III proposed to carry the PW R1830 engine was to be called ‘Scorpion’. Regardless of their individual names, both vehicles remained AC III Cruiser Tanks.