5 June 2017 - Machin 50th Anniversary 1st class Gold Machin stamp from PSB
The 1st class gold definitive was originally issued in 1993, remporarily replacing the orange stamp. This new stamp is from the Anniversary Prestige Stamp Book. There have been many printings of this by different printers.
This is not a new stamp in the SG Concise Catalogue level, but may be listed in the Specialised Catalogue.
In the picture, an earlier stamp is on the left, and the 2017 one on the right.