Check a little closer, maybe with a strong light and a magnifying glass- - - -I'll wager someone has forced the nut to turn and sheared the cotter pin off flush with the spindle. If there's a castle nut on the spindle, there has to be at least one hole for a pin. Later model spindles usually have two holes, slightly off-angled to each other so the nut can be turned a little and pinned without going another full slot on the castle nut.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!