The 2021/2022 Ganzsachen Katalog says this (very rough translation due to less-than-perfect OCR):
Quote:
A common means of protection against counterfeiting was the watermark, which was usually pressed into the not yet dry paper mass using molds during production. While the watermark on the unprinted back of stamps is easier to identify and therefore the MICHEL stamp catalogs show the watermarks from the back of the stamp, the watermarks on postal stationery envelopes are easier to identify on the front. For this reason, the MICHEL postal stationery catalog shows the watermarks as they are seen from the front.
There is either a self-contained watermark image on a postal stationery (Hamburg envelopes) or, more commonly, a sheet watermark that runs across the entire length of paper or cardboard (Bavarian postal stationery 1876-1903). If the sheet watermark is a mark of the paper factory, it is called a factory watermark (German Empire, postcards 1894-1911).
A distinction is made between standing, lying, rising and falling watermarks. Irregular = unintentional position of the watermark is common, so one should not attach too much importance to such deviations.
In the Deutsches Reich section, it then says this:
Quote:
Since July 1894, the printed control data on the bottom right and the dot gaps in the 4th line of the simple 5 Pf cards have been eliminated; instead, the card now has a watermark that indicates the paper mill with a letter and the year of manufacture with a number. The dot gaps in the first two address lines on these cards now mean the month of printing. Since June 1900, all printed control data have been eliminated, but the watermark (letters and year) has been retained on the simple 2 and 5 Pf cards and introduced as a diamond watermark for all cards in 1906. In this, the Roman numeral means the paper mill, next to which the year was also indicated until 1909. From 1914, the watermark and thus all control marks were also eliminated.
Quote:
Introduction of watermarks as control characters:
Instead of the print data and letters that could no longer be exchanged in rotary printing, from 1894 onwards a watermark consisting of letters and numbers was used, initially in one line next to each other (vertical or diagonal), then in two lines below each other; from 1906 in a diamond-shaped border (see explanation before P 73).
The numbers indicate the year the cardboard was manufactured, the capital letter indicates the paper manufacturer. The following letters are used initially:
A (only colored postal order paper), B, C, D, E, L, M, S, W, Z; from 1899 also K, from 1902 additionally St (double letter).
Individual letters in different types (S1 only vertical):
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Quote:
The other letters show only small differences. The year always appears in two digits (94-99) until 1899, in one digit from 1900 (0-5), in K from 1901 and W from 1904 in two digits (01-05). As a rule, the watermark can be read correctly when viewed from the address side, but the position also occurs reversed (sv), upside down (ko) or reversed upside down (sk); the dot gaps are often irregular. The value of these cards with an abnormal watermark is 2-5 times that of those with a normal watermark.
Thers's then a large table with the possible combinations of watermarks and control characters. Do you have a copy of the Ganzsachen Katalog 2021/2022?