As an aside, we see examples of an old protocol regarding similar types (but different marks) shared by FAA and RAF.
The idea was that FAA had odd numbers and RAF even. So the HAS Mk 1, HAS Mk 3 and the Jungly HU Mk 5 came into service before the RAF's HC Mk 2, which was followed by the QF HCC Mk 4.
I would get out more but I'm locked down!