Evolutions
From the philatelic point of view, the "Machins" are far more complex than the simple design might suggest, with well over a thousand varieties of colour, value, gum, phosphor banding, etc., known. Since the first stamps were issued pre-decimalisation, they exist in both old and new currencies. As postal rates changed, new denominations became necessary; the design has been adjusted periodically, for instance to use a gradient shade in the background; perforations have been changed; and so forth. In addition, for the regional stamps of 1971, regions' symbols designed by Jeffery Matthews were added to the basic design.
Initially the stamps were produced by Harrison & Sons using photogravure, with the high-value designs being larger and engraved. Starting around 1980, The House of Questa and Waddingtons Security Print also took up Machin printing in order to keep up with demand, producing their versions via lithography.
Apart from the many values of normal-sized stamps, there have been two different formats used for "high-value" definitives. In 1969 a larger and more square format was used to issue stamps of 2/6, 5/-, 10/- and £1 face value, and was used again in 1970 for the decimal currency values of 10p, 20p and 50p. (The £1 stamp had the lettering re-designed in 1972 and was re-issued. This version is usually seen as a 'decimal' edition as opposed to the 'pre-decimal' stamp.) In 1977 a taller portrait format was used for the large £1, £2, and £5 stamps, and also at various times between 1983 and 1987 for the odd values of £1.30, £1.33, £1.41, £1.50 and £1.60. These values were withdrawn after the introduction of the "Castles" high-value stamps of 1988.
In 1989, as a workaround to the problem of fast-changing rates, "non-value indicator" (NVI) Machins used textual inscriptions "1st" and "2nd" to indicate class of service rather than a numeric value. The following year saw the first commemorative adaptation of the design, with the classic William Wyon profile of Queen Victoria appearing behind and to the left of Elizabeth, marking 150 years of British stamps.
1993 saw the introduction of both self-adhesive stamps and elliptical perforations on the lower vertical sides of them, the latter as a security measure.
On the high value stamps, "Iriodin" ink was used to give them a shiny appearance and ensure the difficulty of their reproduction.
In February 2009, security features were increased on "Machin" self-adhesive stamps to avoid the reuse of uncancelled used stamps retrieved on mail. Both the effigy and the background were printed with continuous "ROYAL MAIL" iridescent printing. Two ellipsoidal shear panels were added to each stamp and the water soluble layer between the stamp and the adhesive was abandoned. These two later features were intended to render the stamps difficult to take off mail and to store for reuse (but in effect are easily overcome by the careful use of a sharp knife edge). Collectors are advised to not attempt to soak such stamps off, but to save them on pieces cut from the envelope.
The security features also included a minute change to the background printing of "ROYAL MAIL" where one letter is replaced to identify the source of the stamp. For example, instead of "ROYAL MAIL" in one place in the upper right of the stamp is printed "FOYAL MAIL" to indicated that that stamp came from a booklet of four stamps. This feature made it easier to identify the source of an individual, used stamp and track down production problems.
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