The NATO rank equivalence poster was a joke with all the RAF OR ranks undervalued, with an RAF J/T shown as an OR2, the same as a conscript with six months service in most of the air forces. I know the Italian and Greek technicians passed out of technical training at least OR5.
The RAF had nine OR ranks, so why not just number them OR1 to OR9, on the chart an RAF CPL is the same as a USAF Senior Airman at OR4.
https://en.wikipedia.org/wiki/Ranks_...orces_enlisted