first off, the Letters were not kept and were destroyed I inquired many years ago on this very subject.   Vann Cremer was the gentleman who was the Historical officer for Australia Post and I agree  a very knowledgeable person, who loved the work he was doing.,   I contacted  him in 1986 about the postal history of a NSW country post office .  and was able to loan him an early negative  which he  copied and gave  back to me with a lovely 8X10 print.   Lorette NSW        I agree with your previous responders: that to the best of my knowledge, they will have been destroyed. Before they sold the Sydney GPO to make it into an international hotel, the historical section was located in that building, and was run by a wonderful man who loved his work and had just so much knowledge, and lots of Post Office historical publications and items. He'd have been sure to give you an authoritative answer.  Hopefully there will be someone equally knowedgeable and dedicated in a similar role today, to assist you. Ray