Oh yes, Douglas did have some ISDT entries, as well as the International Vase. I copied this into the files a few years ago:
Douglas ISDT entrants in the thirties:
1930, France-Germany
#22 H.M. Hicks ‘Class D’
#23 Mrs L. McLean ‘Class B’ (almost certainly a 350cc SV)
#34 W.C. Anstice ‘Class D’
#37 J.W. Douglas ‘Class D’
1931, Italy
Apparently no Douglas entries
1932, Italy
#102 M. Green 600 Douglas
#111 A.M. Martin 600 Douglas
#122 E.J. Kehoe 600 Douglas
1933, Wales
#68 J.L. Park 496 Douglas
No entries for in 1934 or ’35. Did not check for later years. (
http://speedtracktales.com)
The included photo is supposed to be the team for one of the ISDT, though the entrant numbers do not match any of the events above. Of interest is the apparent fitting of DT engines in single loop (A31 type?) frames on the middle and right hand machine (left hand machine is a B31 side vale or similar). They would have to be short stroke DT engines to fit! I inquired to Morton's Media a few years ago about tracking down a copy of the original print, but its has been lost.
A side view of a 1931 Junior TT Douglas shows how it could have been done.
There was also an article by Jeff Clew in the May 2007 issue of Old Bike Mart showing JW Douglas on HY555 (possible the right hand machine of the trio) on a trial over Hard Knott Pass.
There are other pictures of trials teams, but I don't know if they are ISDT, the Vase, or domestic events.
-Doug