The JDH was the fastest and most expensive model Harley produced at the time. In addition to being fast (reported to ride 100 mph right off the showroom floor) it was produced in limited numbers.
This is the first and only photo I have seen of a Harley JDH being used by the police. It looks pretty close to stock other than the exhaust. In 1929 Harley offer a four-pipe exhaust on all twins. The exhaust in this photo appears to have only two mufflers.