Ok, matching that plate in the frame should be a high place on the slide rail , which will fit into the "gap" where the plate part of the rail isn't. Then pull the slide back and turn the plate 90 degrees to put the frame rail back in place. If this doesn't work, then I am really puzzled. However, you didn't say the slide was a replacement. Still, if it is a Model L slide it should fit.
Jim
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