I think that’s it, you’ve found it, it’s Shuma-Gorath. It’s really close chronologically, it’s in a very popular publication, and also in the oldest depictions I could find it also sports something like the “chitinous plates” the beholder description has. According to Wikipedia too, Shuma was/is one of the most popular kaiju-type villains in Marvel. It also says Shuma can levitate and shoot rays off his eye and tentacles.

Eyeballs on tentacles/eyestalks were much more popular back in the day than now, especially for alien or extraterrestrial designs. So there isn’t a huge leap from Shuma gorath to Beholder.

I was hoping for more sci fi or fantasy fans to crop up and point at other characters from 60’s or earlier novels, however, no luck so far. Someone pointed a beholder wouldn’t look out of place in a Lovecraft story and I agree but yours was the only concrete example so, congrats, enjoy your trophy!