My understanding of DA 1st position threaded cog differences from the standard threaded cogs has to do with running a 12t on a DA freewheel and 11t (I think) on Dura Ace freehubs.

To get there with a threaded cog the threaded diameter needed to be a smaller diameter than standard Uniglide.

Shimano 600/Uniglide never adopted the Dura Ace UG freehub, it just moved to HG.

John