In the 60s and 70s Marham based Victor Tankers used M**** callsigns for flights outside the UK FIRs. On Major trails the Tankers were allocated callsigns M***A, M***B, M***C etc, but reserve aircraft were allocated Station Callsigns such as ABC 69. The M**** callsign was tied to a particular task with it's associated Flight Plan, Dip Clearance and Altitude Reservation. It was essential that that callsign did that task, the crew or airframe were irrelevant.
In Aug 68 two Lightnings deployed to Toronto for the Airshow. There were 7(?) Tankers with M**** callsigns plus 2 Airborne Reserves and two Ground Reserves with Station Callsigns.
During the prestart checks aircraft would be found to be u/s, so, when informed of this the Lead Tanker Captain would instruct (for example)" M**D become ABC69, 69 become M**D."
This occured several times, as, if the original Tanker became serviceable again they would swap Callsigns and tasks back. At one point one Captain became confused and was heard to transmit " Tanker A - 69. I am now, oops, 69" The same would occur if changes happened after take off. On this trail the second Ground Reserve arrived at Goose Bay.
On another trail the second Ground Reserve, at Marham, arrived at Gan via Akrotiri and Masirah, with a very sweaty co pilot in an immersion suit.
On a trip from Goose Bay to Offutt AFB the copilot got confused by the Canadian Flight Plan Form and transposed the callsign and aircraft type. ATC were puzzled about the type MAMPA, but the callsign VICTOR1A worked a treat as VICTORIA.