Hello Norm,
thank you for telling us your interesting war experience and for the picture of your stunning collection.
I am not a big expert in mines but after the mine detector had been invented the Germans started to produce glass, clay and wooden mines to make them harder to detect. (Shu mine, topfmine etc)
I bought my No.III mine detector in the UK last year from a guy who runs a used carpentry machinery business. He found the mine detector among a closed-down saw mill equipment that he bought. The saw mill owner tried to use it for detection of metal object in trunks shortly after the end of war but it did not work properly so they hid it in a shed where it had been laying till last year. I think the No.III detector was simply not efficient enough to penetrate thick wood.