It is a strange but true story, the 1974 Proof Set was not struck until 1976. The Mint made a set in 1971 and then stopped. In 1976 they decided to issue all the sets that they had not made, 1972, 1973, 1974 and 1975. That is why these Proof Sets contain coins not issued for circulation.
In the 1974 Proof Set the 50p, 5p and 2p coins were not struck for regular circulation. So if you need these three coins, the only way you can get them, is by breaking up the Proof Sets.
Each 1974 Proof Set contains the 50p, 10p, 5p, 2p, 1p and 1/2p. The 50p, 10p and 5p are now downsized and they don’t make the Halfpence any more. It is an important set that most collectors don’t even know about. All sets will have some toning, but then again they are over 40 years old.