Animal Treasure Island (1971)

Continuity mistake: When Gran first dangles down from the ventilation shaft, the slaver's key ring is too wide to fit through the opening. But when Jim loses his balance and Gran is jerked back into the cell, the key ring is suddenly much smaller than before.

Trivia: For those who wonder about the sign at the lake drainage mechanism: it is actually the original Japanese wording of the warning spelled in Latin alphabet.