no explanation, or even mention of it in CAP413.
Urrr. Well...
There is a clue there - a big one.
CAP.
"Civil Aviation Publication"
Is Shawbury civil? Then why look in a CAP for an explanation?
It is military terminology. Why would that come as a surprise? What does it matter what they call themselves? The service is what you're supposed to be interested in, not the blooming callsign!
Ever heard the designation "Clutch?" Maybe no longer used, just another example.