This is the 1/4" brass pipe plug that seals the auxiliary port on the body of the Bane-Clene® #20860 Strainer, the 1/2" brass in-line solution strainer that protects your pump and jets from grit, scale and tank debris. The strainer body is cast with an extra port for gauge or drain installations. On most machines that port is not used, and this plug is what keeps it sealed under full solution pressure. It is a small part with an outsized consequence: a weeping or missing plug means a pressure leak on the supply side, and you will chase that symptom through the pump and regulator before you think to look at the strainer.