A nice registered cover use of Scott CO1, ACSC 144(OS) from Melbourne to Sutton Coldfield, England. This stamp has a special note in ACSC, "Unlike other OS overprints, the 6d airmail issue was placed on sale to the general public and so could be used for normal postal purposes." Scott adds a similar note, "Issued primarily for official use, but to prevent speculation, a quantity was issued to public distribution." According to ACSC, there were 75,000 stamps issued.
The issue date was November 17, 1931, but this cover was registered on November 19, 1931, so ever so close to a First Day Cover. There are interesting cancellations and receiving marks that provide a bit more information.
Entire front:

Stamps:

Entire back:

Interesting stamp on back apparently showing airmail delivery to the BGM (Birmingham?) Airfield:

There is also some type of cryptic handwriting ... probably by a previous owner, in the top middle, as shown:

It appears to say, "Crash xxx xxx to London". Any ideas on the entire wording? Thanks in advance.