This is the early or first type 20 pence. This reverse design was first used in 1982 on the first (modern day) 20 pence. It was last issued in the first half of 2008, after which the design of both sides changed. As you can see, the reverse is dated, the obverse in undated. We have created an info page about these on our 24carat.co.uk website, following the recent hysteria about the completely undated mules.