Hi Wolfgang,
some more info about the two KNIEPSAND/KALKGRUND:
http://www.sylt-2000.de/Informationen/Zoll/Zollkreuzer/zollkreuzer.html
A good visible difference between those 2 ships is that the older ship has one a smaller bridge window on starboard side.
1986-built: https://www.shipspotting.com/photos/gallery?imo=8747264&viewType=normal&sortBy=newest
1996-built: https://www.shipspotting.com/photos/gallery?imo=9109067&viewType=normal&sortBy=newest
Brgds, Manfred

Thanks Aleksi, you are right. I changed the data, although the period as KNIEPSAND and the latest change into KALKGRUND are not fully clear from the internet.

Following received from klausg: "Hello, the data of the ship written by photographer is not right - length 27m, beam 6m, build year 1986 by Fr. Schweers shipyard (later Luerssen Bardenfleth) as customs cruiser KNIEPSAND (until 1996), later KALKGRUND (until 2010), then sold and renamed GEO OCEAN I, now OFFCON KALKGRUND. It was the first ship of this class. The following ships are 1,5m longer."