I haven't, no, Rob. It's not something I've really looked at enough to give a definitive answer. I don't know when the practice started in Australia.
The races at High Beech on 19 February were usually with four riders but over three or five laps. If Cyclone has the programme for 7 April he may be able to say what the format was for that meeting.
Helmet colours were certainly used at Belle Vue's first meeting on 28 July 1928 with red, white, blue, yellow, pink and green being used in the 6 man races and red, white, blue and yellow in the four man races.
Something else to look into.