Security Measures on the Penny Black

To avoid defrauding the Post Office of revenue it was agreed that security had to be a major consideration. James Chalmers of Dundee reiterated his idea of labels which could be cancelled to indicate payment, some suggested envelopes and others, wafers or labels. It was decided that for security, the background was made of white-line machine engraving as used on banknotes and supplied by the printers Perkins Bacon & Petch. In addition, each stamp had its own 'small crown' watermark. After one false start when the die did not transfer well, a new die was engraved. For extra security Hill decided there should be variable lettering on each stamp, 'A A' to 'A L' horizontally in the sheet, and 'A A' to 'A A' vertically. There were 240 impressions as 240 pennies made £1. To prevent their re-use a cancellation was devised – the Maltese Cross – which was to be used with red ink. Unfortunately it proved possible to remove this without damaging the label, so within a year the colour of the printing ink had to be changed to red-brown and the cancellation to black.

Source: http://postalheritage.org.uk/page/pennyblack