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Just below the word "pushes" in Peter Monta's photo of the Celestaire Astra is a little chromed screw with a flange on it which engages with a groove in the main arc. My guess is that this screw takes out the sideways movement of the index arm against the arc. Now if this screw was a little loose, there would be some sideways play and this would show up as an apparent backlash on the tangent screw. Even if this screw was not loose, there would have to be some working clearance and there would be some apparent backlash in the tangent screw. As you change the direction of rotation in the tangent screw, the first part of the reversed rotation of the screw is taken up with moving the index arm sideways before it starts to move it along the arc...