Has an AFL game ever ended 51-20?
Exactly once. On September 8, 1900, Fitzroy beat Sydney 51-20, and no AFL game before or since has ended that way. When it happened it was a scorigami, the first 51-20 in league history.
That makes 51-20 the 3836th most common final score out of 6,607 that have ever occurred in AFL. One point away, 52-20 has happened never and 51-21 has happened 1 times.