I am looking at the Ninon Valin discography in Record Collector 48 no. 2, June, 2003. The authors are Floris Juynboll, Richard Bebb and David Mason.
On page 110 they discuss Pathe's transition from hill and dale to lateral cut discs and the introduction of the electrical system and, apparently, a shift from the master cylinder to the wax disc recording medium. This is sometime in May, 1927. From 1926 through 1930, some items were also recorded and issued as electrically recorded vertical cut discs.
What is unclear is, when the electrical disc masters were made, were the lateral masters recorded on one machine simultaneously with the vertical on another with different matrix numbers, the same matrix number which, if so, must have confused the part schlepping crew no end, or was one copied from the other.
Aka Steve Smolian