I can't swear to it, but a good guess would be the first performance of Tippett's song cycle, "The heart's assurance," which was given in 1951 (most likely) by Peter Pears accompanied by Britten.
You take the corner of the page, and pull it gently to the left. This will result of you being able to read the back of the page and the page next to it. Then, you will repeat by taking the corner...