PISC Discussion Forums
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I am pretty certain that Horst's xyx is a result of a dry print (normally either under-inking or where some of the ink has not "stuck" to the original stamp). I've got a very similar 5c on 23c with the left hand bottom part of one of the "x"'s missing. I've dug it out and posted a scan. These are not constant errors. Mine is from Row 2, Column 8.
The stamps produced where the printing machine has been low on ink can look spectacular, the overprints fade in and fade out over a block (or sheet). I've included a scan of an example of this wishy washy overprint.
Bryan Jones mentions the stamps with part of the overprint missing in his excellent book on these issues and points out that they are more common on used stamps than on mint. I guess that suggests that sometimes the ink did not always stick to the stamp too well so little bits of it are liable to flake off. Pardon my use of technical terminology there! Horst's stamp looks to be one of the ones where a bit of the ink has flaked off (or never stuck in the first place).
These overprints are produced locally in Fiji. Post Fiji are interested in having stamps to use for local mail. As is evident from some of the stamps that have been issued, they don't insist on aesthetic perfection or uniformity when it comes to the overprints.