I may have found the build date I was looking for. A bit of digging in the Folding Bikes forum and found another member posted this.
As you probably know, that's a Sturmey-Archer factory date code, May 1986. Back in the day, the hub would have been shipped from GB to Taiwan and subsequently built into a bike some months later.
The venerable Sturmey AW hub has three speeds, and that gave this model Dahon its name, Classic III. The Classic V had a five speed derailleur drivetrain, the Classic I was a single speed.
Mine is also a SA 3-speed with "85-9" stamped. So I'm going to say the bike is a Classic III and probably built in late 1985.