Solomon Islands 1978 20 cent
A number of coin reference books make reference to a 1978 Solomon Islands 20c struck with no FM mintmark. 1978 Solomon Islands 20c pieces struck by the Franklin Mint are well known - 544 were struck for mint sets, 5,122 were struck for proof sets and 6,000 were struck for matte sets, though none were struck for circulation. Searches by the author are yet to yield a 1978 Solomon Islands 20c with no FM mintmark however.
If such a coin were to exist it would most likely have been struck by the Royal Mint in Llantrisant. It was there that all circulating 1977 Solomon Islands coins were struck, as well as the circulating coins of 1981 and 1985. The Royal Mint Museum has advised that they have no 1978 Solomon Islands 20c in their collection, so it is safe to assume that they did not strike the coin, and by extension, that the coin likely does not exist.
A likely explanation for the listing of the 1978 Solomon Islands 20c with no FM mintmark in catalogues is a probable small run of 1977 20c pieces struck in 1978 and listed in the 1978 Royal Mint Annual Report.