At least one holding lug (24) on the baseplate (22) is coupled to a bolt (34), which engages with a recess (28) in the baseplate when enclosing the slat edge. The recess can be at one end of the baseplate and clear of its edge. One or two lugs (24) protrude at right angles from the intervening portion (26) of the baseplate and over the recess. When the portion (26) is distorted, the bolt engages with the side edge of the recess.
