It APPEARS that some of the edits came in 1966. CBS was scheduled to show Psycho on its Friday Night Movie show on Sept. 23, 1966. A murder in suburban Chicago (still unsolved) a very short time before the airing date caused CBS to shelve the showing out of sensitivity to the family...
I think the "R" was given to the story content, which at the time WAS considered "edgy" and controversial. The Peeping Tom, the murders, the shower scene (while not explicitly violent sure implies a lot) and the cross-dressing were all pretty intense storylines sixty years ago.
If I'm not mistaken (and someone WILL correct me, I'm sure!) two scenes were trimmed, the shower and Arbogast's death. The second scene apparently has much more impact. I believe it's not a huge difference in running times.