This much, I can answer:
Was orbital resonance just not a conceived concept until the sixties?
Laplace recognized orbital resonance long before then, and he died in 1827. I looked, couldn't find a precise year of publication.
Now, 3/2 resonances were perhaps suggested/discovered quite a bit later, I'm not sure.
Some mathematics on the subject in the following link, a little bit over my skill-set, but it makes the point of stable resonance.
Also, the link points out that the the first resonance discovered, Io, Europa, Ganymede the 1-2-4 resonance, have ratios of 2.0076 and 2.0143, or, 0.4% and 0.7% off resonance.
I'm tempted to say that small variations like that fall within a stable zone, but I'd basically just be guessing. I can't say for sure.
Pluto was discovered in 1930 and Charon, it's rather large moon, not discovered till 1977. Perhaps in the 1930s, Pluto was no longer cutting edge physics and all the big brains were looking at Quantum physics, Hubble's discovery and other galaxies, implications from Einstein's discoveries, such as black holes and Nuclear bombs . . . but again, I'm only speculating. It does seem that a discovery of the 3/2 resonance should have come before 1965 and I think it's a good question.
Not to take this too off subject, but I love the story of how Neptune was discovered. Short summary here: http://coolcosmos.ipac.caltech.edu/ask/146--When-was-Neptune-discovered-
Longer one here: https://en.wikipedia.org/wiki/Discovery_of_Neptune
My favorite potential orbital resonance is the Jupiter/Mercury one, which could, eventually, pull Mercury away from the sun.
The Jupiter/Mercury resonance is described as "coincidence" in the article, where as, Pluto's 3/2 with Neptune and Jupiter's 3 inner moons are (I think), probobly not coincidental resonance but some form of stable orbits.
Hope you don't mind my amateur/hobbyist answer.