Douglas Myall published the news yesterday that the non-denominated security Machins have a hidden code that identifies the format in which the stamp was issued. In other words, just by looking at a single stamp, you can tell if it came from a counter sheet, business sheet, booklet of four, booklet of six, booklet of twelve, or mixed definitive/commemorative booklet.
The code is a change of one letter in the curvy ROYAL MAIL pattern, located above the diadem.
The codes are as follows:
B replacing A in Royal - ROYBL MAIL - in business sheets of 50 (large 1st and 2nd Machins) [corrected April 19]
B replacing A in Mail - ROYAL MBIL - in business sheets of 100 (standard 1st and 2nd Machins) [corrected April 19]
C replaing A in Mail - ROYAL MCIL - in mixed booklets with four 1st Machins and two commemoratives ("C" is for custom)
F replacing R in Royal - FOYAL MAIL - in booklets of four stamps (large 1st and 2nd Machins)
S replacing A in Mail - ROYAL MSIL - in booklets of six (standard 1st Machins)
T replacing A in Mail - ROYAL MTIL - in booklets of twelve (standard 1st and 2nd Machins)
There are no hidden codes in counter sheets, which means that no denominated Machin has a code.
Royal Mail says that these codes let them know the source of a stamp when a problem is discovered, for example, a sub-standard application of the phosphor bands.
Myall is giving each of these stamps a separate number at level 3 in his Deegam Handbook. Users of the Handbook who have signed up for electronic download of his Deegam Reports can download the latest, DGR 82, at his web site, http://www.deegam.com/. This issue shows the various codes and includes his catalogue listing and profiles.
UPDATE: Some images are here.
If you need a refresher on these new Machins, click here and here. Also this lengthy discussion on the Machin forum.