Quite possibly made by the Handley Watch Case Company, based in Abbotsford, Melbourne, although it doesn't have the Handley stamp.
(J. W. Handley Watch case Manufacturers, 655-7 Victoria street, Abbotsford.)
They made watch cases for brands like Rolex, Tudor, Omega and many others.
Handley went out of business some time in the 1980s I think.
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