I think that you will find that the first actual Reg 203 licence was indeed issued to Opal Air in about 1967. The story was summarised as the "cover story" in Aviation Safety Digest number 89, published in 1974 (ahh....remember them).
I have my copy to hand and it clearly states that the first such licence was issued to Opal Air for their Cessna 402 operation, although the date is not mentioned. The earlier Cessna 206 and 310 operation was conducted under a charter licence.
Now I know that gaunty is not often wrong, but appears in this instance to be forgetting that there is more to Oz than just the Sandgroper state!