We reported earlier what appeared to be two different types of London Underground self-adhesive booklet stamp, as the dark blue 'caption' line seemed to be larger on some than on others. True comparison was not made easy by the (what have become) usual poor standard of registration on Walsall-printed boookets.
The white numbers are achieved by leaving unprinted spaces in the printed area of the grey, magenta and cyan, a technique known as 'reversed out'. Clearly any misregistration (as on the left example) leaves a date with coloured edges, in this case grey, but regular readers will recall the magenta/white/cyan (or red-white and blue) face value on the 1st class Jubilee booklet stamp.
According to Royal Mail,
printers were concerned that the blue was a very tight fit with the
reversed out text and could result in a white border showing. They
used varying heat in the printing stations to adjust the print width
of the blue and this has caused the variation in the two images –
too much heat and the blue shrinks, too little and it spreads. Not
ideal but preferable to a white border.
And the reason why the mis-registration occurs in the first place is that the
job has to go through the press twice and the heat and tension causes
the paper to move.
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