The Series D was fitted with Armstrong dampers front and rear, but a very different unit for both, the front damper was very well suited to the Girdraulic fork, but had to be used with longer bottom eyebolts than the Series C damper, although sometimes the Armstrong damper shows up in for sale adverts, they will be 65 years old, and to the best of my knowledge are unrepairable or serviceable.
As Marcus says a Koni will do the job, they did manufacture a damper specifically for the front forks which was a little softer than the rear unit, documented in KTB, but even that is normally considered a bit too hard in use to get the best Girdraulic fork action.
Works Performance was good, but no longer available, Spax had a try, but one or two snapped the damper rod in use and consequently was withdrawn.
So you are left with AVO, which is very good, readily available, and, pretty much made to any specification that you require as far as firmness is concerned, and has adjustable damping in situ, which is convenient, it is the damper that is supplied with the Emmanuel/Walker fork mod, but is also available for standard set up Girdraulic forks.
Or newly manufactured Series C damper, or an old one with newly manufactured updated internals available from VOC Spares or Maughan and Sons.
I have been using an AVO for the 6-7 years without any complaint or issues.