More on Synchronizing Mickey

In this Readers Digest (August 1934) version of an article from the July 1934 Woman's Home Companion, we find on page 108 a simple description of the bouncing ball system to record Steamboat Willie...We also find mention of "the three raids on Disney's staff."
Now - the first was Mintz' Oswald take-over in early 1928, and the second was Pat Powers' Iwerks/Stalling maneuver in January 1930.
But what was the third that this article refers to?