Thanks for your response..it appears to me that just the knob is missing,however i am not at all familiar with this sight. At present someone has placed a small lock washer and brass nut in place of the knob. The elevation slide moves freely up and down but I don't see a little sliding nut that you refer to behind it. What's the purpose of the sliding nut..it seems that the locking knob would keep the elevation slider firmly in place. If it would be helpful I can try and post a few pictures later today. Thanks ever so much for your interest. Art
