LUTHER - How far IH planned ahead depended on what they were planning on doing. If they were planning anticipated production of a NEW item, the Marketing Dept would come up with some numbers via a SWAG. Sometimes the plan was WAY too low, sometimes way too high. For some reason the Marketing people always seemed to mis-read the market, reacted badly or too late to conditions. IH was heavily driven by their marketing people and I could make a REALLY strong argument that they were the cause of IH's downfall, not the union or poor upper management.
For production of existing product with a decent history they would plan out the current month plus twelve more, total of 13 months. In most cases depending on the item the current month plus 2-3 coming months would be firm, no changes allowed. Long lead-time items like engines, major machined castings, etc would be in this group. On small items, hardware, small simple steel stampings, decals, lights, etc the firm schedule would only be current month plus the first month.
I'm not exactly sure when the 44A & 50A decks were released, sometime late in the 1X8/1X9 production, 1972, maybe 1973, I'm pretty sure they didn't build decks 4-5 yrs before starting to ship them. I'd actually be surprised if they started making parts for them to replace the old 42" & 48" decks even 4-5 MONTHS before the demand required. They would have made a pilot run of parts and assembled them to test the tooling, but the final Eng Change releasing them for sale wouldn't have been done to put them in production until they got engineering & marketing approval for the design until the decks were done being field tested.
As complex as IH's manufacturing system was, they actually had a VERY well managed manufacturing control system, new parts, engineering changes, etc all very well documented & planned.