Thanks for sharing this letter. I have (somewhere) classical issued CBS Cassettes that had the double ring on them just as the Quad Album had. Also, the Reel to Reel issues of Quad Albums I have obviously was the same recording. On the "Introduction to Quadraphonic Sound" the narrator talks about Quadraphonic tapes.
You may be thinking about the Angel double circle .
I believe Columbia Records had no indication on their cassettes.
I did find six Masterworks SQ S.I. on an lp stuffer . One was "Three Penny Opera".