For some bizarre reason Yandex tossed up a link on a search for something somewhat related to an entry in the IWM catalogue which is the range tables for the 21cm (and larger calibre) guns with the note the folder contains 6 handbooks of the Wilhelmgeschutze. The IWM has scanned some of the images which show the firing platforms and various other interesting items. The handbooks are not (afaik) available on line but they appear to be a treasure trove of Wilhelmgeschutze information.
