While working for Agusta in Italy....during a visit with their AOG section....I noticed the clerk typing a response to an Operator in which the Clerk informed the Operator that the requested part was out of stock and that the next scheduled manufacturing run for that part was six months away.
That was back in the late 70's.
Part stockage is not an easy thing to do as some parts rarely get requested while others are always in need by someone.
Having parts on different aircraft models that are common to more than one aircraft makes it somewhat easier.