Abstract: Three rows of feelers sliding in openings in support plates each bear either a blocking element with a single sight for the outside rows of feelers or a blocking element with two sights for the middle row. When all the feelers are in operative position, the various blocking elements define two parallel sight paths which permit a light beam conveyed to the entry of the first pass through one of the sights of the middle row blocking elements and through all the sights of the blocking elements of one of the outside rows to be reflected from two mirrors, disposed at right angles to each other, and to return by passing through the other sight of the middle row blocking elements and through the sights of the blocking elements of the other outside row. The light beam is conducted from an emitter to the entry of the checking device and from the exit thereof to a sensor by fiber optic cables.