The camera is a Sputnik.  For those who don't know, it's a Soviet era
Medium Format stereo camera.  It takes six 6x6 stereo pairs on a roll
of 120 film.  Although they are notorious for various problems, this
one has been tuned, does not leak light, and the shutter, aperture and
focus are all matched.  The film is fed through the camera from right
to left, so odd frames are the left frame of a stereo pair, and even
frames are the right frame.

I just picked up two rolls of E100G from the lab with a very strange
problem.  I suspect a problem at the lab, but I can't think of what it
could be.

On the roll that I shot first the first six frames (i.e., the first
three stereo pairs) are fine.  Frames seven through nine (the fourth
stereo pair and the left frame of the fifth stereo pair) are
completely clear.  The edges of the film are fine, so I can read the
frame numbers and the manufacturer's markings.  There is no separation
between the frames.  Frames ten through twelve (the right half of the
fifth stereo pair and the sixth stereo pair) are also fine.

On the second roll (which I shot after the other roll) about 3/4 inch
across the leading width of the film and the edges of the film are
fine, but the entire image area of the roll, all the way to the end,
is clear.  There are a couple of spots that have very faint images,
but other wise the film is clear, again with no frame separation.

The two shutters on the Sputnik are linked together.  The left shutter
has no timing mechanism and is driven by the right shutter.  I could
imagine the link breaking in such a way that the left shutter is stuck
open, but that doesn't explain why the last three frames of the first
roll and the first 3/4 inch of the second roll are fine.

The camera is at home, so I can't check it until later.

Does anyone have any idea what would cause this?  I've seen just about
every problem you can have with a Sputnik, but I've never even heard
of anyone having a problem like this.