Edward T
With the law suits going around on narrow aisles products built without the rear OHG posts and later added as standard feature, I'd be careful of viewing the OHG as only protecting against FOPS. You are correct in what you state about OSHA/ASME requirements.
In a real life situation, a large glass manufacturing company I was working with, used a forklift in the 'cullot" (melted used glass) room, without an OHG. The forklift was fitted with a "truckers" mast and only need to lift totes 6"-12". However, there were several metal beams that extended down from the ceiling about 4'. The operators had to drive around them like a down hill skier on a slalom course. Yep, an operator managed to back into one of those beams with is head & back. Yes, they were cited for a lot of things - one being on OHG to protect the operator. Of coups, the courts awarded damages (small by comparison to today's standards).
This was in 1977/78 and OSHA and lawyers were just getting warmed up.
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