Recently my 67 has developed an issue with what seems to be the curtain. The electrics are fine, mirror functions as it should and MLU, it fires correctly on every speed. The problem occurs when advancing the film. Whilst advancing (dry firing) the second curtain isn't pulling across, so I'm left with an open frame. The light obviously then comes through the lens, creeps under the mirror and instead of the curtain blocking the light until i'm ready to fire, the light is fogging half the negative. I've tried the reset button, different combinations with the reset button. Checked there's nothing preventing the curtain from moving.
Any suggestions? I've attached a few images to show the issue.