The Unit I shuttle is a unique way to pass film through the gate for
exposure. It has fixed pilot pins that register and then hold the
film in place during exposure. This procedure is repeated in an identical
manner each time, allowing for the film to pass through the camera
several times for exacting multiple exposures. The model 2709 soon
became the industry workhorse with by far the longest working career
of any camera I know of. Bell & Howell 2709's are still being
used for doing time lapse, and are working in the field, even today.
I know of a Canadian shooter who loves his, and uses it to shoot the
Aurora Borealis in winter, while shooting on location in Canada, the
U.S and Mexico in the summer. Granted he has modern computers and
electronic stepper motors driving the camera, but, the camera body
and movement is still the Bell & Howell 2709. If he manages to
contiue shooting until the year 2012 he will have helped make the
2709's working career 100 years long. I don't know of another type
of machine that has a working career of that length.