I'm curious about possible differences between the response of learned societies and of commercial journals (though I understand that a few societies were already making enough money from their journals by the 1960s that there is no hard-and-fast line). For commercial journals (Nature, Science, the rising number of Pergamon journals), a system that appears to remove the need for journals by providing a faster route to circulation, was a clear commercial threat. But for those societies which were still predominantly mission-focused, the speed of communication within the scholarly community might have been welcomed - a way of recreating a community remotely. What strikes me is that this is just at the time when peer review was starting to become used more widely - beyond the society journals. I've seen evidence from the Royal Society from late 1950s that the Society was starting to present refereeing (done by its fellows, as volunteers) as the unique advantage that society journals had (at that time) over commercial journals (or at least, certainly over the rapid-publication weeklies, e.g. Nature does do peer review as standard till 1973, see M Baldwin). Thus, I could see a possible line of argument for disciplinary societies, that preprints would provide a useful speedy notification service, but would not detract from the value-added by waiting some months for the referees of a society to assist in creating the final version of record. (Or, they might worry that pre-prints would facilitate the circulation of non-refereed stuff, and thus de-value the society's editorial processes...) Either way, I was curious that refereeing, and the lack of it, does not seem to come up in the debates you describe...