Has an AFL game ever ended 51-21?
Exactly once. On June 14, 1958, Carlton beat Richmond 51-21, and no AFL game before or since has ended that way. When it happened it was a scorigami, the first 51-21 in league history.
That makes 51-21 the 3836th most common final score out of 6,607 that have ever occurred in AFL. One point away, 52-21 has happened 3 times and 51-22 has happened 2 times.