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At the same time, Nikon produced these AI normal lenses: 50/1.2, 1.4, 1.8, 1.8 E series, and an ƒ/2, a 55/1.2 for a while and the NOCT; most temporally overlapping the others in production.
Perhaps they divided the optimal performance over several lenses for a reason? Technical? Expansion of lens sales depending upon different needs? Makes designs simpler since each one had its specific role? Who knows. Different design philosophy.
As a rule, Nikon's had a 50/1.2 or 55/1.2, a 50/1.4 and either a 50/1.8 or 50/2 in production at any given time, there's no production crossover on those (but there was availability crossover). Essentially there was the budget model (1.8/2), the everyday model(1.4) and the specialist model(1.2).
The 50/1.8 E and Noct existed outside of that line, one as a cheap lens originally matched to the EM consumer body (although all the Nikkor 50/1.8's except the original AI and the recent G are in fact derivations of the E design) and the other as a specialist lens at a great cost (you could buy all other Nikkor ~50mm's for less than the cost of a Noct)