"FrBoot" design, 1940
I had no idea what this small, shallow-draft vessel was built to accomplish before receiving an email from Jaroslav Tvrdy from the Czech Republic.
The Russian markings label it as a river minesweeper.
Jaroslav offered this detailed information, with minor edits:
These were river minesweepers (Flussraumboote) built for German Danube flotilla. Even though the design is dated 1940, twelve of these ships were built by Lürssen shipyard in Vegesack, Germany, in 1938- 1939 and numbered FR-1 to FR- 12. The plans were drawn by Naval engineers, not by the shipyard. They were armoured and carried at the beginning one small-calibre MG in the turret, replaced later by the MG 151 20mm cannon or by a MG 131 machine gun. Three were lost by the end of 1941, followed by another three in 1944-45. The remaining surrended to US-Army in Linz, Austria on May 8, 1945 and were later rebuilt and used as tugs.
There is no evidence that another batch of these minesweepers was intended to be built after 1939.
The collection of the river warship plans (WW II) from Linz Shipyards is said to be confiscated by the US Army and should be in an American archive.
I'll point out that this one drawing (at least) came through Soviet hands, and may have been from a German shipyard. If the Americans might have more in their archives, it's also possible that more might be in BAMA in Freiburg, where most of the assets confiscated by the Soviets were eventually repatriated in the 1990s.