I believe QG's correct that the rear shutter and lens shutter are out of time. The lens shutter should be completely closed before the rear shutter opens.
The sequence is:1)release button pressed, 2)lens shutter closes, 3)rear shutter opens and lens shutter makes exposure, 4)rear curtain closes when shutter button is released.
What you are not seeing, but should, is the shutter closing before it releases.
You can test the lens by removing it from the camera and releasing it. The release is a small pin in a semi circular tube next to the cocking axle. just press it sideways not inward. You should see actions 2&3 above.
If I remember correctly, the timing for this is controlled by the gear in the shutter but it has been far too long since I've worked on these. I think that if it was the cocking shaft that you wouldn't be able to mount the lens without damage.