http://www.kmw2-modellbau.de/flusi1.html
I will provide my edited version of a machine translation of the information from that site (I am not fluent in German, either

"Krischan the Grosse was built in Rotterdam in 1940-41. The idea was to create a simple small vessel, which with its strong anti-aircraft guns could control the air space in the canals and along the Dutch / French coast. However, trials revealed some problems in the design and construction of the originally planned 100 copies were never started!
One point was that the main weapons only had two axes of movement, which is also normal for land weapons, stabilization at sea, however, is complicated. Furthermore, they had no way to slow the engines! Thus mooring was carried out at high speed, which obviously was extremely complicated and dangerous!
The armament consists of two 88mm Flak 18, a 37mm Flak, and a 20mm Vierlingsflak, as well as several depth charges."
I'm not sure I have interpreted the bit about the engines correctly, as it does sound quite odd.
Basic data was as follows:
Length: 38.50 m
Beam: 5.90 m
Draft: 1.50 m.
Displacement: about 300 tons
Maximum Speed: 18knots
According to the forum posts, the vessel used BMW B4 petrol engines with 87 octane aviation fuel, in contrast to C3 with 100 octane (?). The posts also say that details and technical data for Krischan the Grosse (Flusi 1) and the slightly different design Flusi 2 of 1942 can be found in:
GRÖNER Erich, German Warships 1815-1945, Volume 7, Page 213
There appears to be some uncertainty about the fate of Krischan the Grosse, which appears to have been destroyed on 24.2.1944 in / near Bergen op Zee by air attack with 11 dead.
If anyone else can improve upon or correct the above information, please do so.
David
Edit: I forgot to mention I had previously encountered mention of the vessel on a Russian website, of which I have unfortunately lost the URL. The Russian site mentioned that the aft 88mm originally fitted was later replaced by a second 20mm Flakvierling, as shown in your photos.