Pontius
The clock I was speaking of ran from the date the risk of not meeting the 85dB(A) exposure limit could be reasonably eliminated, with a period of grace to allow for implementation. One would have to look up when the 85dB(A) limit was introduced.
Many such 'legal' requirements were only taken seriously when individuals lost Crown Immunity in the late 80s. As I alluded to, there was a very conscientious Group Captain who managed an Applied Research Package, but he got precious little support and when he retired the RAF lost a 'champion'. It was pure luck that the RN's first formal requirement to mention the limit as an 'essential' (Sea King AEW Radar System Upgrade) was endorsed just before he left, and he was tracked down and his files transferred. As it happened, no bidder was compliant (some suggesting the main gearbox be redesigned), so the MoD(PE) Directorate took it on at risk (using underspends/offsets). The trouble with this approach is that you get the naysayers queueing up to criticise you for wasting money - as they did, frequently. £758 per helmet mod set. £500k+ compo, and more to come? Do the arithmetic.
Hitherto, as you say, hearing loss was well-known, and presumably someone deemed it tolerable and ALARP - because there was no engineering solution for aircrew using a helmet, primarily because the electronics were not small enough to fit in an earshell. For example, Racal had a pretty good system (but not as good as MoD's), but it didn't fit. (They also supply a very simplistic broadband ANR for army vehicles, but that is unsuitable and unsafe in an aircraft).
The point I'm making is that the award to the serviceman will have been based partly on when the solution was available, and what MoD did about it. That was the legal advice to us in the early 90s. The practical problem was, OR would go to the scientists at Farnborough (superb), who would produce reams of reports full of mind-numbing decibel notation. OR would topple. The AEW programme used a different ploy, expressing it as 'allowable flying hours'. The s**t hit the fan when this was calculated to be a mere 59 hours per year (and the subject of a Jane's article), the alternative being to give an 8-aircraft squadron several hundred aircrew. Questions were asked in the House. £758 per helmet mod became a no-brainer, even to beancounters.