I joined the RN as a helicopter pilot in 1967 having spotted the loophole which allowed HSP with 6/18 vision. - John Eacott
I was never quite sure what the exemption was based on - MPN11
By no means an authoritative answer, but I would hazard a guess that the "loophole" or "exemption" arose around the time that General List officers of the Supply Specialisation (now the Logistics Branch) were being encouraged to volunteer for service as helicopter pilots, with a consequent variation in the required visual standards, and presumably applied to all specialisations until the door was subsequently closed.
Just for the benefit of our light blue colleagues, I would add that the Supply Specialisation and the Royal Marines both produced helicopter pilots who attained four stars, long before the light blue caught up......
Jack