18 mm barrel, cylinder-position inductive proximity
The XS618B4MBU20 is an 18 mm diameter inductive proximity sensor from the Telemecanique XS range, classified under cylinder position inductive proximity sensors — the 18 mm barrel is a standard form factor for detecting piston position in pneumatic or hydraulic cylinders, where the sensor mounts in a threaded port and senses the cylinder's internal magnet or a ferrous target on the piston.
Sourcing and cross-reference context
No official successor or replacement part number is recorded from Schneider Electric. Peer parts in the same category include the XS9E111A2L2 and XS4P30AB9D2, both inductive proximity sensors from the Square D / Telemecanique line — but the XS618B4MBU20 is an 18 mm cylinder-position variant, while the XS9E111A2L2 and XS4P30AB9D2 are different form factors (the XS4P30AB9D2 is a 30 mm barrel, for example). The XUYAFPCO946S is a photoelectric sensor, not a direct inductive substitute.