That's really interesting, thanks. I never did see any available for sale. Did they actually work , i.e. decode in a stable fashion?
I just remembered a few more matrix encoded cassettes that were made available.
There were a large number on the "Sure "label all SQ encoded . (mostly Philidelphia mummers ) Quad Incorporated Discography has the complete listing . ...as likely does Mark Anderson's Quadraphonic Discography
And my Columbia records 76 catalog supplied to me courtesy Joe Dash at Columbia's quad promo division,
Listed one Stan Kenton on the Creative World label and one on the Doric label . (there were likely others on those 2 quad single inventory labels )
Also I believe Project 3 ventured down that highway but titled their tapes as being "wraparound " sound .