Thanks for the info Chaps, I suspected the s/a is symetrical- its true to say there`s not much room for any offset anyway. I noticed when attempting to measure mine that its no use using the end faces of the pivot tube as a datum- they point inwards slightly due to the distortion of the tube when the arms/corner braces were welded on.Distortion that cannot always have been the same both sides.I can only assume BSA bored the tube for the bushes after welding to achieve (bush) alignment. I suspect that during production, manufacturing tolerances did wander away from the drawings somewhat, and pictures of workers trueing up frames in fancy jigs to 0.0000whatever was probably more sales pitch than reality......... Cheers, Bob