Does the IIa have a vertical or horizontal shutter? I had a similar problem with my Spotmatic. The Spotmatic has a horizontal shutter, and at the fastest speed, the distance between the leading and trailing shutter was small. The leading shutter was dragging a bit, so the trailing shutter would close the gap as the shutters moved across the film plane. At the top speed, the trailing shutter would catch up with the leading shutter about 75% of the way across, so the remaining 25% wouldn’t get any exposure. That would leave the left side of the frame (the right side of the image) would be black.
On a vertical shutter you could get a similar effect, but it would be the top portion of the image.