I have a feeling it may be a Darnell boat, going by the ballast system and country of acquisition.
However Darnell never made a 'D' class submarine AFAIK. In fact the only WWI era hull I know he produced was a K-class, and they were very limited quantity (single figures production run I understand).
British classes he did produce include S, U, T, P classes and X craft. For Germany he did two variants of Type VII, XXI, XXIII and biber.
Most of the boats were built toa 1/40-1/50th scale, with the smaller midget and coastal subs built to larger scales between 1/6th for the Biber to 1/32nd scale for the Type XXIII.
A final note, An E-class boat was produced by British modeller Dave Diplock. This was available for a few years as a hull kit or as a complete kit with ballast system from Craycraft. I can't recall the exact scale, but I think it was 1/32.