This is a Maytronics Dolphin D-Type Shaft Hexagon Drive Adaptor for use on many Dolphin pool cleaners.

Some of the Dolphin models are Advantage, Deluxe, 3001, 2002, 2x2, Heavy-Duty, Enduro 11r/21r, Pro, Pro X2, Commander, Interactive, Plus Batttery, Orion lI, Advantage, Explorer, DX3, DX4, Wave 100, Dynamic DX5 Plus, DX5, DX6, Deluxe 3/4, Supreme M3/M4/M5, Nautilus 115V, Wave 50, Apollo, Endeavor, Atlantis Basic, Triton, Edge, 2x2 Gyro, Saturn, Discovery, Wave 75, Advantage Plus, Wave 65/90, Apollo Plus, Bio PVC, Deluxe 5 Trevi, Evolution X5, Primal X3, C3, C4, C6 Plus, C5, M400, Nautilus CC Plus, M200, Apollo Windo, C3 Dynamic, Triton Plus, Neptune and Neptune Plus, Edge Dynamic, Apollo Plus, and Wave 200. Also works on the Pentair Prowler 820/830.