Given the following reasoning and answer, what was the question? The important part for answering this question is: He achieved an unmatched historic treble when he won the Grand National in 1973, 1974 and 1977, and also came second in the two intervening years (1975 & 1976).
The answer: 1973
The question Passage: Red Rum (bay gelding; 3 May 1964 -- 18 October 1994) was a champion Thoroughbred steeplechaser. He achieved an unmatched historic treble when he won the Grand National in 1973, 1974 and 1977, and also came second in the two intervening years (1975 & 1976). The Grand National is a notoriously difficult race that has been described as "the ultimate test of a horse's courage". He was also renowned for his jumping ability, having not fallen in 100 races.

Question: Based on this passage, when did red rum win the grand national?