Australia 1951 PL Sixpence.

The 1951 PL sixpence had the coarse milling common to all Australian silver sixpences. If your sixpence has the fine milling, it would be of the same fineness as the British copper nickel sixpences of 1951.

It must be remembered that the 'PL' mint mark means that the Australian sixpence was struck in London. In 1951 the mint was have producing British copper nickel sixpences and Australian silver sixpences.

The milling on the the copper nickel coins was finer than the milling on the silver coins. Advance forward to 1966, (the year of the introduction of decimal currency in Australia), and the copper nickel 5 cents has the fine milling of the British copper nickel sixpences.

It seems apparent that a fine milled collar die for the British copper nickel sixpence must have been used for the silver Australian sixpence, instead of the coarse milling collar.

How rare or common?. I don't know, but it is possible that the production run may have extended for as long as the obverse / reverse die set in the coining press lasted. I have never seen a fine milling 1951 PL sixpence, but I have never looked for one, either.

Perhaps the literature on this subject may may be found in articles in the Australian Coin Review Magazine.