Extremely rare,  this DR70 Bell and Howell camera was converted to a pulse camera in the 50s to balance helicopter blades by Chadwick Helmuth.  The camera is in excellent shape and would be a rare addition to a collection though I would think could still be used in the trade but I wouldn't have info on how to use it as I am not an engineer just a fellow camera collector.   The term "Chadwick your helicopter" is still in use today.   Background below.

Slip-Sync technology evolved into many applications, including photography of human vocal cords and capillary blood flow, design of orange-grove antifrost-circulation fan blades (which eventually replaced the “smudge pots” of the early 1930s), and a complete system used to track and balance helicopter main blades and tail rotors. Balancing eventually extended to propeller blades, including the props on the world-circling Voyager.

Vibrex™ systems are now the main product of Chadwick-Helmuth and are used worldwide. 

Chadwick’s original design work was done in a small shop attached to the home he built more than 54 years ago in Bradbury, CA. The original Slip-Sync used an N-6 Gun Sight Aiming Point (GSAP) wing camera from World War II fighter planes. Converted into a pulse camera by Jack Urban, another early SPIE member, it eventually was replaced by a Bell and Howell DR70 spring-wound 16-mm camera, modified in Chadwick’s shop. A pulse camera capable of reliable operation up to 30 pulses per second was a breakthrough in the early 1950s.

Chadwick’s Slip-Sync system was first used at the Jet Propulsion Lab (Pasadena, CA), documenting vibration tests of spacecraft components. It replaced the previous method—rolling a small barrel of instruments down the cement front steps of the lab.

As Chadwick traveled the world selling the Slip-Sync technology, he encountered customers who needed to balance helicopter tail rotors, which were failing because of excess vibration. The Slip-Sync system solved that problem, leading to a high-output strobe light, which could be used to track main rotor blades in the daylight.