the fibers of green cotton only recently became commercially available when a long-fibred variety that can be spun by machine was finally bred.
in this above sentence, I think the word "when" introduces the sufficient condition (according to LR Bible) and "only" introduces the necessary condition, so the logic condition is: long-fibred variety that can be spun by machine (sufficient) --> green cotton became commercially available.
However, my this understanding is apparently wrong based on the answer. So I wonder whether my understanding of the logical is a mistaken reversal
the long-fibered variety isn't enough to be sufficient. it's hinting that it might be necessary, but it's not quite there. for it to be sufficient, it'd have to be the case that, given a long-fibered variety being bred, fibers of green cotton would be the inevitable result. we don't have enough evidence to say that.
the word 'only' doesn't have anything to do with necessary-ness or sufficiency in this question, though. it sounds like you might be taking some of the examples in the LR bible a little too seriously -- these words can introduce certain types of conditions, but they don't always have to. I don't remember this question at all in terms of what the answer was, or what kind of deduction it was looking for, but I'm guessing it was asking you to be able to understand that the cloth being machine-spun isn't a necessary condition for green cotton -- it's just trying to point you that way with some leading langauge.