I've no idea where Billboard got that 'story' from but there's not a single scrap of evidence to support it over here where it was supposed to have happened and the activities of the BBC were (and still are) closely scrutinised and documented.
Why did the likes of Angus McKenzie never mention them (even after the event?). Why did the BBC not publish the results of such tests or mention them in their own study reports? Why did no BBC engineers ever talk about them?
Why was it so secretive you ask, well for obvious reasons. Not as secretive as you would have been led to believe if BBC Radio Times was your only source of info, and as my (now deceased ) friend and journalist........Alan Turner managed to record some Classical works in SQ matrix.
He offered them to me after he gleaned all the rock , pop, plays, that I wanted from his 22 Reels of H and HJ broadcast. Unfortunately all his BEEB SQ was Classical---so I passed on that.
So sorry that you were not in the loop , as he was.
And too bad he did not have a like of rock or pop or I would definitely of availed myself to that offer.
"Blind " matrix tests have to be so . Simply put..... A PLACEBO effect , which is the only true means for establishing both mono and stereo compatibility.
This you know was done and I showed you the secret H broadcasts from August 76 Proms.
Additionally secret was the Norrie Paramor album in H ....on ....the ....BBC ...Record label, no less!