I'm guessing the mirror is being released early, the shadow is way to indistinct to be from the edge of the metal shutter curtain on the Copal Square shutter. Those cameras had a motor drive available, and the extra wear could be the cause of the problem. See:
http://www.mir.com.my/rb/photography...s/fm/part5.htm There is a pdf file of the timing of the mechanisms you can look at.