Via Barnstormers New Zealand. I'm not entirely sure about what it is said about this being a special 1930 motor. The sloper engines were used in several years in the 1930's. But it is true that they did not last long. This dovetails with the photo discussed recently in royalenfields.com
Note: Barnstormers' site is temporarily down. Here is a cached version of the article. Hat tip David Blasco.
The Motorcycling Du Ponts
17 hours ago