Andy,
The part number for the rear fork bush on the 350 and 600EW to take the brake operating arm is 7954-1. For the S6, like the example Eddie showed, the part number is 7954-3. It is the same part that has gone through two minor revisions (-1 to -3), but was still considered interchangeable. If not, Douglas would have assigned it a new part number. 7954-3 also fits the A28 (1928 version of the EW350), B28, C28, B29, and L29. The were still using the same part number in 1938. So the dimensions Eddie provided should work for your model.
The washer is 7955 and the nut 8348, and was shared by all models with the servo band brake.
-Doug