2) The clue seems poorly worded -
The prototype photofinish camera - based on the notion of photographing moving objects through a slit onto a continuous strip of film - was first used to decide placings in the Grand Metropolitan Handicap at Epsom on 22 April 1947: the winner Star Song was a length to the good, but the judge needed to consult the photo before deciding that Parhelion was second, a head in front of Salubrious.
In the 1948 season a photograph was called upon to decide the outcome of horse races on 119 occasions, and the same year photofinish technology was used for the first time in the Olympic Games, held in London.
The following year a photo was first called upon to decide the outcome of a Classic race in England when Nimbus beat Abernant a short head in the 1949 Two Thousand Guineas at Newmarket. Later the same season it was first used to determine the outcome of the Derby, when the Judge called for the evidence of the camera before announcing Nimbus the winner by a head from Amour Drake, with Swallow Tail another head away in third.