Hi Sandy,
Big Bertha, 42cm morser in 1/76th scale was printed in Military Modelling May 1975, try ebay that's wear I found my copy, if no joy PM me and I will try to get a copy over to you.
Thanks for these plans! They seem to be drawn from the Paris Army Museum model...? I have used them to make a SolidWorks version (3d model) of the Big Bertha, you can find renders on this forum. (Sorry, not sure how to link to an existing post?)