I guess the U 991 operated mostly on the Norwegian coast having the norwegian naval mainbase at Horten. The original paintings would probably be close to the U 995 se photo below of U 995 May 8th 1945 Trondheim. U 991 was in the days before the surrender transferred from Horten to Bergen and under operation Pledge transferred from Bergen to Scapa Flow arriving June 4th. 1945. This convoy consisted of U-218, U-530, U-778, U-875, U-907, U-991, U-1004, U-1005, U-1057, U-1271, U1301, U 1307 and U-2328. Altogether 13 U boats out of about 100 surrendering in Norway. The transfer was done by the German crews and they were indeed instructed to fly a black flag. She later was sunk under operation Deadlight . Her only war patrol lasted about 73 days and from Kristiansand, Norway to Bergen, Irish Coast, French coast and return to Bergen a patrol pattern I have done many times on board U 995 during the months of October- December. Unfortunately for the weathering, apart from one time our fwd. stb casing was smashed, we never experienced much of a "weathering" in these waters, but as you certainly have both the skill and interest for weathering why not? Below is the last photo of U 995 under german WW2 command CO Hans Georg Hess, alongside the submarine pen in Trondheim, Norway May 8th 1945.
As an extra, U-995`s last and youngest ever CO of a german submarine, Hans Georg Hess interned in a war prisoncamp in Trondheim, Norway.
Tore