that are to be coupled together while the third transmission means 5 c serves as the means that optionally establishes the form fit of the clutch and is moved to and from to the side u2014 or radially for this purpose fitted on the driveshaft 1 a axially alongside the drive sprocket first transmission means 5 a is the second transmission means 5 b which is mounted rotatably with respect to the driveshaft 1 a in the illustrated exemplary embodiment which is preferred in this respect said second transmission means 5 b is a sprocket which can be identical to the drive sprocket in terms of outside diameter and toothing the second transmission means 5 b bears on one of its end sides a linear drive coupler also referred to as connecting rod means 11 below which converts the rotary movement of the transmission means 5 b into a linear movement its contour in the side view illustrated is preferably crescent shaped via an extension means 12 which has two members in the exemplary embodiment an adjustment means 20 known per se is actuated and as a result of its movement in particular pivoting movement u2014 as is known per se u2014 one folding shutter element in the exemplary embodiment the upper folding shutter element can be locked and unlocked and can be folded up out of its.