An apparatus includes a latch (42), an armature structure (64), and a spring structure (66). The latch (42) is movable from a non-locking position to a locking position in which the latch (42) engages a seat belt tongue (16). The armature structure (64) conducts magnetic flux from a magnet (62) to a Hall effect device (60). The spring structure (66) biases the armature structure (64) into sliding contact with the latch (42) during movement of the latch (42) between the non-locking and locking positions. The latch (42) affects the magnetic flux conducted by the armature structure (66) so as to cause the Hall effect device (60) to have a first output when the latch (42) is in the non-locking position and to have a second, different output when the latch (42) is in the locking position.