This 13 minute film (courtesy of the British Council Film Collection) shows BBC School's Radio as it was in 1943.

"Lessons from the air".


At 5.13 in, the narration refers to "'this ingenious device' (78rpm FX disk / chinagraph editing)  
which "allows the producer to select just that part of the disk....."



Original Description
School Broadcasting
'This picture shows how school broadcasting enriches the curriculum and brings into a little village school a breath from the world outside. It gives an idea of the central organization, showing how closely the designers of the broadcasts are in touch with schools.
There are many classroom scenes and shots of B.B.C. Talks Studios.'
(Films of Britain - British Council Film Department Catalogue - 1944-45)

Great stuff.