A sensor assembly is provided for a fingerboard latch assembly that comprises: a latch bracket; bolts for mounting the latch bracket to a fingerboard; a latch; and a bracket pin rotatably supporting the latch on the latch bracket to allow rotation of the latch between an open position and a closed position. The sensor assembly has a mounting arrangement that mounts to the bolts, holding a closed-position proximity sensor probe facing downwardly for sensing proximity of a crank portion of the latch and/or the piston head in the closed position, and also holding an open-position proximity sensor probe facing forwardly for sensing proximity of an arm of the latch in the open position.