Has an AFL game ever ended 31-15?
Exactly once. On June 19, 1897, Geelong beat Fitzroy 31-15, and no AFL game before or since has ended that way. When it happened it was a scorigami, the first 31-15 in league history.
That makes 31-15 the 3836th most common final score out of 6,607 that have ever occurred in AFL. One point away, 32-15 has happened never and 31-16 has happened never.