The dates don't fit.
In the 60s with V-Force QRA and the National Deterrent the BBC Light Programme would have become a public service broadcast, I can't remember the name Peter Hennessy said it was.
What we expected to hear was:
"Codeword is now in force."
There was little difference from what was broadcast 20 years previously. I have no idea if that was a single message or whether other codewords would have been broadcast as well. That one codeword would have both authorised our strike and declared which target set was in effect.