Am I missing something? I'd have thought that even if the fixed jaw is not vertical, after machining all four sides you would have the same error between each plane resulting in a square squared block????
You could also try contacting the German national archives:
http://www.bundesarchiv.de/service/kontakt/index.html.en
But, I'm not sure that a detailed model of an 88mm would be any easier than an aero engine!