Likely the right hand margin slider has shattered. I just fixed this same issue on my SG-3 by 3D printing a replacement using a free file available on Thingyverse. My library printed it at no charge. The repair took a little doing— I had to drill out two small brass rivets and replace them with tiny screws, but it is great to have it working again.
via u/radiant_control at https://www.reddit.com/r/TypewriterRepair/comments/1xk6tkj/olympia_sg3_right_margin_stop/