The Treasure Box (1979)

A pornographic filmmaker dies, and his last will and testament is shown to his survivors in the form of a self-made film. In the film, he tells them that he is going to show them a porn "loop" (short film) that contains clues to the location of his fortune, and whoever solves the mystery and finds the hidden treasure will get his entire fortune. In this way, the lucky winner will avoid paying an inheritance tax.