5 June 2017 - Machin 50th Anniversary 20p bright green from prestige book
This gummed stamp with elliptical perforations is a quite different stamp to many of the earlier ones.
This is not a new stamp at SG Concise Catalogue level, but may be recognised in the GB Specialised catalogue.
There have been different printings of the 20p with elliptical perforations. The picture shows one of the earlier ones on the left, and the latest one on the right.