Not sure without knowing the stem diameter and length between the bearing caps. Most girder forks were either Webb or Druid although some manufacturers made their own. Could even be that Triumph and Ariel used the same fork. Given that the military used a lot of Norton in that period, they are another possibility.
I have a lot of pictures from the Barber museum that show a similar girder on Nortons, Triumphs and Ariels so that's not really any help; but, the one you show us does have the same front brake fixing as this so that may be a valid clue:
__________________
1964 Highway Trophy TR6SR
2004 Bonneville America