An interesting production error that escaped quality control at Thomas de la Rue (which was printing these banknotes at the time).
The fifth digit from the end in both numerators is unobscured. It can only be seen in outline, as shown in the attached macro photos, which at the same time show the absence of any manipulation and the full naturalness of the area of both of these unembossed digits.
With today's fully electronically controlled printing machines, the creation of such an error would no longer be possible, but at the time the numbering machine used at Thomas de la Rue was a SUPER NUMEROTA II machine made by KBA - the pinnacle of technology at the time - but nevertheless set fully by hand.
Such an error, as we can see on the present piece, was the result of an error in the synchronization of the numerator with the printing (ink) cylinder. The number itself is also not insignificant - the last one in the cycle before the backward running numerator jumped. At the same time, this banknote printed with an error, was probably the extreme in the file, possibly obscured by an additional band, so manual quality control did not notice it, thus letting it pass into circulation.
Such errors were possible with the production technology of the time and on the aforementioned machine.
It was a fully manually set machine, which, in the case of printing e.g. 200 zlotys, involved the manual setting and connection of 80 numbering machines before the start of each production cycle (40 banknotes per sheet), and all sorts of errors did happen, however. Hence the multitude of replacement series for banknotes printed on these machines, not only at Thomas de la Rue, but also later at PWPW.
At the time when these series were printed, quality control was still manual, so some errors sneaked into circulation, like this piece.
A rare and interesting item - definitely recommended.
As a curiosity we can add that the effect of such a human error in setting up the above mentioned machine was the banknotes, which had the bottom number differing by one digit from the side number, and whose appearance on the market some time ago electrified collectors.
