1955 Rhodesia and Nyasaland Proof Set
Proof sets of the first issue of Rhodesia and Nyasaland coins were issued in 1956 - the Central African Currency Board had asked in 1955 if the coins could be struck in silver rather than curponickel as the circulating coins were cupronickel, and the Royal Mint was able to oblige and strike the proof sets in .500 silver (p57, Hodgson, The Rhodesia and Nyasaland proof set of 1955 in Coin News, March 2023).
Although all 2,000 sets were struck in .500 silver, a number of catalogues list 1955 cupronickel proof sets as having been struck as well, though this is not the case. The origin of this error is unknown, but individual cupronickel proofs were struck for institutional collections (p58, Hodgson, The Rhodesia and Nyasaland proof set of 1955 in Coin News, March 2023)