I don't know . I'm still miffed that they went S I so very fn late !! Why not sooner ,if your Matrix Encodes really are stereo compatible . They should of had the brass balls to back up their own investment and SQ invention. That's a sure fire way to convince the record buying public that quad is the real deal for the future of music.
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Add to that last (Classical) quads labels list single inventory from the likes of ; Eurodisc , Harmonia Mundi , Supraphon ,Peter's International ,VOX/Turnabout/Candide , Vanguard, Sonic Arts , RCA , JVC., Nimbus .and (probably Sony).
I'm not too certain when Acanta , Bellaphon , and EMI/Electrola quit with their releases ,but late 70's makes sense.
FWIW in the late 70's, I used to see plenty of Harmonia Mundi SQ albums, fr/Spain , France , Germany ,and I think Italy?