For postcards of the German Empire the cardboard was supplied by multiple sources and in order to keep a check on the quality, a system of dots was used from 1876 onwards, which enabled tracing of the source. Initially a gap was added to the left-hand or the right-hand end of the 4th address line. From 1879 onwards the system was enhanced by removing dots from either the left-hand or right-hand side at the 5th, 15th, 20th, 25th or 30th position on the 4th address line. From 1890 the dot position also corresponded to the letter following the print date at the bottom right-hand corner, e.g. 5th dot = 'a', 15th dot = "b" etc.
From 1894 the control data for the 5Pf cards was changed from dots to a watermark. The watermark also contains data for the print factory and date of printing. The month of printing is determined by dots in the first and second lines of the address.
For the 2Pf cards the paper factory could initially be determined by dots in the lower right-hand corner, however this was dropped in 1900 when all control data was removed.
From 1906 until 1914 the watermark was used to determine year of printing as well as the paper mill (Roman numberal) for 2Pf and 5Pf cards.
Note there are also gaps in the address lines that do not appear to have any meaning.