In the east Bay Area of California, in the early '60's, we called flip flops key-thongs. (The spelling is likely wrong as I couldn't read at the time.) We moved to New Mexico in the late 60's, where they had no idea what that meant. Does anyone remember the term? Was it Bay Area dialect? Where did it come from?
I have lived in the East Bay (Berkeley and Albany) of the San Francisco Bay Area since 1984, and I've never heard the term key-thongs used here. Nor have I ever heard a pier or a dock or a wharf or a boardwalk referred to hereabouts as a quay.
Of course, 1984 is about 20 years after the period cited in the OP's question, so it is certainly possible that key-thongs was a short-lived term for what most of the United States knew as flip-flops or thongs. (In southeast Texas, where I lived during the 1960s, the rubbery footgear was known first as thongs and then, rather abruptly in the early 1970s, as flip-flops.)
This evening I asked a lifelong resident of the East Bay (Albany and El Cerrito), now in his mid-sixties, whether he had ever heard of the term key-thongs, and he said no; when I explained that it referred to what are now called flip-flops or thongs, he said that the people where he lived called such footwear flaps. No one else I've asked in the area recognizes key-thongs either.
Maybe key-thongs was a term specific to Livermore and its environs (assuming that that's where Livermoron is from), or maybe it was even more localized than that, or maybe it had so brief a heyday in the East Bay that longtime residents no longer remember it. In ny case, the theory that key-thongs is a homophone for quay-thongs is severely undercut by the fact that quay is an unused and largely unknown word in the Bay Area. For that reason, the sense of quay-thongs would have been lost on most people who live here, and it's hard to imagine why it would have arisen in this area as opposed to elsewhere.
A search of Google Books yields no instances of key-thongs or quay-thongs in the sense of flip-flops/thongs, but it does turn up several mentions of a mythical beast called a keythong. From Arthur Fox-Davies, The Art of Heraldry: An Encyclopædia of Armory (1904):
English armory knows an animal which it terms the male griffin, which has no wings, but which has gold rays issuing from its body in all directions. Ströhl terms the badge of the Earls of Ormonde, which from his description are plainly male griffins, keythongs, which he classes with the panther ; and probably he is correct in looking upon our male griffin as merely one form of the heraldic panther.
Marc Epstein, The Medieval Haggadah: Art, Narrative, and Religious (2011) shows an illustration of a keythong opposite a wyvern:
FIGURE 11 Cloven-hoofed, Wingless griffin (also called an alce or a keythong in later heraldry) symbolizing the sun, and two-legged dragon (wyvern) symbolizing the moon.
All in all, a very cool animal. But not something you'd want to slip your foot into.