From your pic, the freehub has both UG (external) and HG (internal) threading and should accept either type of cassette. The UG cogs do not need to be aligned to fit and can go on in one of 9 orientations (18 if you flip the cog). HG cogs need to align the fat tab on the cog with the fat slot in the freehub and can go on in only one orientation.

Dura-Ace 7400 hubs had a different freehub attachment method than other Shimano hubs and accept only Dura-Ace freehubs. But I don't think a Dura-Ace 7-speed HG/UG freehub was ever made. Dura-Ace was 8-speed by the time Shimano introduced HG and the hard-to-find Dura-Ace HG/UG freehub is 8-speed. If your freehub is only 7-speed, I don't think it is Dura-Ace. If your hubs are Dura-Ace, I have no idea how someone put a non-Dura-Ace freehub on an old Dura-Ace hub - could be it's just held in place by the axle.