The minimums are not the same as in the PHB - in some cases, at least (druids, rangers, maybe monks?, and paladins other than CHA) they are more forgiving.the PC classes are listed with minimums
How is this relevant? It shows that any attempt to find a systematic set of rules governing NPCs in 1st ed, or establishing the resemblances/differences between NPCs and PCs, is fruitless.
And two questions - do henchmen get the benefit of the more forgiving minimums? And if so, does this affect whether or not a player whose PC dies can convert a hencman into a PC? (My assumption was always that henchmen have to abide by the PHB minimums, meaning that the second question becomes moot. But I didn't get this from the rulebooks, which are silent on the matter. I'm sure there were other more-or-less coherent ways to work it out.)