This is the fourth United Kingdom proof mint set featuring the - then new - decimal Pound Sterling currency. At that time, it only had the six circulating denominations included in the set. Larger denominations, and the 20p coin, were issued much later.
The set also includes a 1974-dated oblong Royal Mint token.
All coins in the set (and the set itself) are dated 1974, but it was actually issued in 1976.
This was the only mint set issued for this year. The Royal Mint started issuing Brilliant Uncirculated sets only in 1982.