I was the MoD desk officer in D Air Ops for the Hercules fleet as we signed the contract for the J and then started planning for its arrival. OR 51(?) managing the actual procurement claimed primacy over designations for the J and insisted that numbering was determined by entry into service.
That idea certainly seems to fit in terms of the Ks, thus:
On initial delivery, all short and Hercules C1.
Then we modified one into Snoopy and it became Hercules W2.
Then we stretched a few and called them Hercules C3.
I wanted to maintain consistency and call the short ones C4s, the long ones C6s and leave C5 empty for any special then as yet undetermined role modification. However, OR51 over-ruled me because of the supposed rule and, as we got the stretched
Js first, they became the Hercules C4 and then when the stubbies, the last 6(?), arrived they became C5s.