Not 100% sure but I think the Sigma Derby manuals here might be for the newer updated/electronic version of the machine, not the original mechanical one. The horses are mounted to a base that glides along, sliding on the smooth racetrack surface, pulled on their way by a magnet under the racetrack.
$450 for a pdf of the manual on ebay is kinda out there, can't imagine anyone actually buying one at that price. What an entrepreneur, he can just keep duplicating them on his computer forever. Maybe someone should put an ad on ebay listing it for $10.