In an earlier thread, I posted that americanradiohistory.com was a great resource for old audio-related magazines. New PDFs of magazines are constantly being added. Here's a direct link to the June 1975 issue of Studio Sound and Broadcast Engineering, which is devoted almost entirely to quadraphonic articles:
http://www.americanradiohistory.com/Archive-Studio-Sound/70s/Studio-Sound-1975-06.pdf
http://www.americanradiohistory.com/Archive-Studio-Sound/70s/Studio-Sound-1975-06.pdf