A few weeks ago I mentioned the first of my three columns published by Linn's Stamp News on the occasion of the Machin anniversary. The final two parts have been published, but the full text is restricted to subscribers only. The web site contains on the first section of each column - the second one is here and the third is here.
Linn's is still offering a free subscription to their digital edition, and I think that should get you access to the full text of my columns. You can sign up for the free access here. If anyone tries it, please let us know the results in the comments.
One of the illustrations I included with the column is this side-by-side comparison of the original 1977 £5 Machin (on the left in the image above) and the recent one issued for the 65th anniversary of the Queen's accession. The purpose was to show the difference between the portrait derived from one of the original photographs of Machin's cast and the portrait derived from the 1997 digital image. Of course, the production methods of the two stamps are different - the 1977 issue was produced by photogravure using acid etching, and the 2017 issue was produced by electromechanical engraving (EME), often referred to as simply 'gravure' - but most of the difference results from the new image.