Might well be hyperbole, but this would have been back probably late 19-teens, 1920 at the latest given Black Gold’s birthdate. Were those claiming procedures like today followed way back then even at all little tracks?
ETA: Just went hunting in my library for any other source on this, which I didn’t have a chance to do last night. Blood Horse in the Ten Best Kentucky Derbies says that the incident took place in Juarez, Mexico, in 1916, and involved a double-barrel shotgun. Apparently, per their version, there was an argument in the stewards’ office, and then the owner stalked out and went back to stake out the stall with his gun. Maybe the argument in the stewards’ office took place after an attempted taking over on the track? Anyway, their version is more detailed than Marguerite Henry’s. Owner was ruled off all tracks in North America for life, which indicates that there must have been a pretty major offense of shotgun-wielding level.
ETA2: Skimming that whole chapter on Black Gold, there’s a point I had forgotten. When Black Gold won the Kentucky Derby, his owner, the widow of the above shotgun-wielding “nobody claims my horse” guy, insisted on being paid the purse in cash. She was from a Native American reservation, and she was understandably a bit suspicious of paper promises. Churchill paid her in cash as requested. :yes: