Project 3 was never a single-inventory quad label, they did dedicated stereo mixes of pretty much their whole catalog, maybe with the exception of a handful that were specifically made for quad like that Q4 test record.
Additionally, they didn't start putting out matrix quad LPs until 1971 or 1972, but their first stereo releases were from January 1967 (Enoch Light's Spanish Strings was the first one) so the first 50 or so titles in the Project 3 catalog were available in stereo only for nearly 4 years before they came out in quad.
If CDs are from a matrix quad master it should be immediately apparent when decoded, as all the Project 3 stuff is very "4 corner discrete" with various brass, voice and percussion sections pinned to the corners of the room, and often call and response sections will answer each other from diagonal directions.
The upshot of this is that you can't take for granted that any Project 3 released on CD is matrix encoded - it's just as likely (or probably more likely) that it's from the stereo mixes.
Additionally, they didn't start putting out matrix quad LPs until 1971 or 1972, but their first stereo releases were from January 1967 (Enoch Light's Spanish Strings was the first one) so the first 50 or so titles in the Project 3 catalog were available in stereo only for nearly 4 years before they came out in quad.
If CDs are from a matrix quad master it should be immediately apparent when decoded, as all the Project 3 stuff is very "4 corner discrete" with various brass, voice and percussion sections pinned to the corners of the room, and often call and response sections will answer each other from diagonal directions.
The upshot of this is that you can't take for granted that any Project 3 released on CD is matrix encoded - it's just as likely (or probably more likely) that it's from the stereo mixes.