Most people know that Litho plates or cylinders don't last as long as those used in gravure printing, meaning that for long runs, multiple plates or combinations of plates might be necesssary.
Way back in 1969 Harrisons printed nearly 140 million of the 5d Christmas stamp from one set of cylinders - though apparently they had to have two tries at making the brown. According to Stanley Gibbons specialised catalogue the cylinder numbers are:
1A blue, 1B yellow, 1c magenta, 1d green, 2E brown, 1F light blue, 1G red, 1H gold.
The 271 million 4d second class stamps resulted in two combinations of cylinders:
1A brown, 1b vermillion, 2C orange, 1D purple, 1E blue, 1F or 2F green, 1G violet, 1H gold
In the 21st century few stamps are printed for each issue, and it seems that one set of gravure cylinders or litho plates is usually sufficient. But it would seem that Cartor have had some trouble with Battersea's Dogs & Cats - either that or there are many more millions printed than usual.
We've now had a response from Royal Mail which explains why this problem occurred, and they have provided a full list of the 9 cylinder combinations used.
"The reasons for this unusual range of plate combinations are twofold. Firstly, this issue was printed fine screen litho to get the best translation of the images as we could get. Because of the delicacy of the images on the plates this results in a shorter life for each plate. The second impact was the phosphor ink. At the time these were printed we were having problems with the quality of the phosphor ink on hand. Cartor's attempts to compensate for this resulted in a number of plate changes before a new batch of the ink was received."
Thanks to Martyn for providing this information which is, I'm sure of interest to many collectors.
So far the combinations found by us or reported to us are numbers 1 to 8 in the above list, leaving number 9 still to be found. (Type 3 just reported in a delivery from Tallents House - thanks, Marc.) See comments for locations of some combinations.
Please let us know what you have found; remember you need two sets to include the phosphor and gold numbers.
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