I remember Oakington (Varsity) in 1970/71 doing the planning for an ‘airways trainer’, and having to look up the various crystals we would require, and to tell the flight line engineers which ones we wanted for the trip.
We got airborne and joined airways at (I think) BPK and started along the airway to CLN. After a few mins we were told to contact London on a frequency we didn’t have in the box.
There was a waving of instructor hands and an instruction to acknowledge the frequency change and then to keep quiet.
After about a min. the said instructor calls London, on the original frequency, and says that we had no answer on the new one just given.
The reply from London was a request to tell him the frequencies of the crystals we did carry! All a bit embarrassing.