Clive,
I printed out a copy of this picture and have been looking at it from time to time, but have to confess the steering stem continues to flummox me. I have not yet figured out anything Douglas that quite fits the bill. When Douglas changed the design and placed the lower spindle bolt directly under the center of the steering stem, they spaced the fork blades out much wider than previously. It is possible that someone has cut down such a later stem (circa 1934 and onward) in width to suit the EW style fork blades, links, and springs seen here.
But the upper link anchorage argues against that for several reasons. The 1934 style had adjustable handlebars, the clamps for which grasped either side of the stem, inboard of the links. The forging step up slightly to a machined diameter. Here the forging steps up to make a purposeful larger shoulder for the links to bear against. This suggests the width has not been modified, or at least not by very much. Were it an EW through A31 top forging, one would see the boss on the back side for the brazed on handlebars. For the 1934 style, the back had a pair of lugs for a pinch bolt and it was of course split to enable it to clamp the stem, all of which would be visible. Also it would have the lug for the central compression spring on the opposite side that could hardly escape notice. There is no obvious evidence this forging has been modified.
Of course there are a few models in between the A31 and the 1934 style that it could have borrowed parts from, but I do not think so. The heavyweights like the S6~D31 used a forging where the spindle continued to be forward of the stem, and the T6~E31 continued to use the old style stem of the 600EW (much like the EW with sockets for the headlamp irons) to exhaust stocks. The lightweights as far as I know continued on with the EW pattern till the new design in 1934. I can also rule out Bantam and F/G31 OHV models.
The other thing that suggests it is a bodge cannibalized from other make parts is there is nothing visible to lock the upper forging to the stem to stop rotation between the to. It does not use the long thread of the EW pattern with the bearing adjuster sleeve and jam nut, with the top forging nutted down firmly to that. Nor the split clamp and pinch bolt of the 1934 and later system. Here the upper nut/stem cap can be run down presumably to set the bearing pre-load, but there is nothing further to really clamp the forging rigidly to the stem. Perhaps there are some keys and keyways or a spline hidden inside the upper forging, in which case it certainly is not from Douglas.
So unless I am overlooking some obscure Douglas model, my vote is the stem and top forging are from something else other than Douglas.
-Doug
[technical edit, 22apr08, Doug]