I first heard about this from G3VA. I believe he wrote about this in his monthly RADCOM column Technical Topics years ago. It may also be in one of the histories of the Bletchley Park, Hanslope Park
Going along with "off topic" I have a National HRO XM with a complete set of Coils it came from England and would date from 38/39. It is marked with the number 16 painted on all the coil sets as well