Three Peg Bar
However, already in February 1927, a patent was filed (granted May 1929) that describes the three peg system. It also gives good reasons to implement this system. The inventor named is Frank Lyle Goldman, who according to this page was Max Fleischer's best friend! (If this is the same person, but what would be the odds there were two of that name in animation at that time on the East Coast?)
Those of us who "grew up" using good old ACME pegs, or you East Coast-ers using Oxberry pegs, may have wondered why one professionally would not just use two pegs which seems simplest. Well, here is the explanation. Having manufactured aluminum animation discs myself back in 1983, I must admit that this did not come as a surprise to me. Obviously, the pegbars that we were used to had flat outer pegs (and yes, one could lay a ruler over the outer Oxberry pegs), but the principle is the same as in above patent.
Do we know when Fleischer started using flat-round-flat holes in their animation paper? Did the Disney studio have to pay a licensing fee for using the three pegs in addition to their regular two round pegs?
[Still really busy - not much time for blogging!]