I just visited the museum last week with a friend from England. We talked about this gun quite a bit. It looks like a Naval mounting off of a battleship or cruiser, more than anything else. The chassis its on is a heavy lift linear type, one that could be used to move extremely heavy items for ship building moving in a straight line. The gun is simply welded onto the carriage. Talking it over, we thought it might of been unshipped and used at a dock facility like Kiel or St Nazaire. The sign mentioned that the whole thing weighed something like 18 tons (I could be way off, though) so it would have to be a static mount. It's gotta be really rare!
-Ski