In Australian stamp catalogues only the blue-black overprint is mentioned. There are actually three different versions of the £1 Coronation overprints. Two different Ash printings and the rare McCracken overprint on slate-blue. There has never been a display of all three together, and not many people will have all three in their collection, let alone two of them. All the stamps are certified.
All stamps shown are MUH.
In order of display - Left to Right. Blue-Black (Ash); Grey (Ash) Black with the stamp being Slate-Blue.