I feel reasonably confident that this coin has not
been plated. The extra mass of 1.2 grams is much
more than be accounted for, with plating.
I would also find it reasonably difficult to explain why a white metal alloy coin should be plated with a white metal.
I would expect that XRF testing would reveal a standard alloy of 75% copper, 25% nickel for this coin.
Weight needs to be re confirmed on a different accurate mass balance.
If correct, I suspect that a wrong planchet was fed into the coining press. This happenstance is not unique; it happened (rarely), when The Royal Mint
was executing coin orders for many countries around the British Empire.
happens occasionally, although less often.