This is the seventh 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. There is also a Crown-sized coin, officially denominated as equal to 25 pence (although this is not displayed on the coin itself).
The set also includes a 1977-dated oblong Royal Mint token.
This was the only mint set issued this year. The Royal Mint started issuing Brilliant Uncirculated sets only in 1982.