The Senko 215-103-J1 is a premium 125um single mode ST fiber connector with a blue mini 900um boot.
The SENKO ST connector has a bayonet-style housing and a long spring-loaded ferrule hold the fiber. They are available in both multimode or singlemode versions. Horizontally mounted simplex and duplex adapters are available with metal or plastic housing, with a choice of phosphor bronze or zirconia split sleeve.