All the material I've ever seen also suggests that the modular connectors were introduced mid 1970s.
I have a couple of WE-500 sets dating from 1970/71,and they still have hardwired cords. One of them came to me direct from Idaho and had the old-style 4-prong plug which was color-matched to the cord and phone (a kind of pinky-beige color). The cord actually has spade ends for wiring to a wall junction box and the plug is designed to accept the same terminations for easy conversion.
Some years ago I used to be involved with lighting and sound for amateur dramatics, and I was always on the lookout for things which were out of place in "period" plays. Quite a few of the errors I pointed out to the producers stick in my mind to this day.
There was a play set during the Korean war in which one of the leading characters almost went on stage wearing a digital watch which would have been visible to everybody in at least the front two or three rows. In the same play where a scene had rows of books on shelves, one particularly thick volume had large labeling on the spine which could easily be read at a similar distance -- "Almanac 1971."
Another play set in -- if I recall correctly -- the 1930s had the set mocked up with modern-style light switches which simply didn't exist back then.
On telephones, there was more than one play in which somebody in props had procurred what to them was obviously just "an old phone," and I had to point out that the British GPO 700-series phones didn't appear until the late 1950s.
I was pedantic about the use of the props too. There was one play set in a small seaside town on the south coast in the early 1950s in which a character had to make a call to London. One of the older members of the cast had remembered that there was no STD back then and that a long-distance call had to be placed via the operator, but he then started dialing 100 to place the call. I had to point out that prior to STD we just dialed 0 for an operator. There were people saying "It doesn't matter," but after mentioning this the fellow thought for a moment and then recalled "Oh yeah, that's right, we did, didn't we?" So if he remembered, you can be sure somebody in the audience would spot the mistake too (and not just a telephone geek like myself!).
Most people wanted to strive for accuracy as far as possible, but we had one producer who was "The what does it matter" type, and I had more than a couple of disagreements with her over technical inaccuracies.
You don't even want to know what she thought we could get away with as a doorbell in "Little Women."
-Paul.