Never seen a sleeve like that ever. I wonder where it originated. I doubt the CK Q8 in it came in it originally, but who knows. Maybe Nick Perugini would know, or maybe dr8track or Q-Eight.
That is unusual also considering they use the term "QuadraSonic" Sound, not Quadraphonic. So that kind of narrows the search down a bit. It doesn't look like a Canadian United Artists cover either. I can only speculate that it might be from a secondary or minor label that was trying to get into Quad? Is there anything written on the back?
Yep, it's a definite mismatch. If I had to guess its origin, I would lean towards one of those "Gas Station 8-Tracks". For you young folks , the US Gas Station used to be an oasis on the highway, where long stretches of road throughout the country only had a gas station every 30-100 miles. Those stations were the genesis of the modern mini-mart, but the core gas station used to give customers maps (long before GPS, yes folks used maps) and at the register (no pay at the pump) there were usually a rack of 8-track tapes of dubious origin. Mostly clones, some called them "mob-tapes" or "trucker tapes", created by original pirates. There were actual quad varieties of these tapes, some of you have seen them.
Now that you mention that, Jon - that certainly makes me think this slipcase could be a match for one of the "Sound Values" tapes! Those tapes also feature the wording "8-track Stereo" and "QuadraSONIC" Sound!