The O-17 (TM77) fits onto the white TM connector on the output lead from your AccuMate or OptiMate charger and enables connection to an SAE connector.

This allows your AccuMate or OptiMate charger (or accessory) to connect to the new SAE-compatible range of accessories as well as to the connectors found on some lawn mowers & garden tractors. SAE connectors are also used by some other battery charger manufacturers, but check polarity.