Last night we watched Mr. Moto's Gamble, and it was interesting that Keye Luke appeared on that in the character of Lee Chan, Charlie's son, with many references to same.
The reason that Keye Luke appeared is that this was originally supposed to be a Chan picture called "Charlie Chan at Ringside". Unfortunately Warner Oland had some personal issues which led to him leaving the set which required some recasting/reshooting to turn this into a Moto film.
As far as I know, there is no surviving footage of the Oland version of this movie.