overhanging shelves 104l and 104r extending from the respective rear surfaces the joining bracket 102 thus serves as means to hold the housings 101l and 101r in absolutely fixed position with relation to one another the housings 101l and 101r also have projections 106l and 106r that extend directly toward each other and are bolted to the under surface of the horizontal plate 66 by machine screws that pass through the holes 64 only one of which is shown in fig5 and enter threaded holes 107l and 107r aligned with the holes 64 although each housing 101l and 101r is joined to the clamp foot 25 by only a single screw the fact that the housings are rigidly joined together by the bracket 102 prevents any swiveling of either housing relative to the other or to the clamp foot both of the housings 101l and 101r have threaded holes entering their outwardly facing walls only the hole 108l is visible in fig5 these holes allow detent means to be inserted into the housings 101l and 101r and to be adjusted after they are inserted fig6 shows a top view of the structure 99 ready for a clamp frame to be attached to it each housing has a front surface 101l and 109r and the bracket 102 holds the housings so that these flat front surfaces are coplanar with each other each of the front surfaces is pierced by a hole 111l and 111r respectively that extends straight into the respective housing perpendicular to the front surface.