Yeah, I was surprised by the lack of search results when I was double-checking my post too, but apparently I wasn’t surprised enough, because I was wrong. I mixed up two pieces of Showstopper!: chapter 5 mentions the Win32 spec being initially written in two weeks by Lucovsky and Wood

> Lucovsky was more fastidious than Wood, but otherwise they had much in common: tremendous concentration, the ability to produce a lot of code fast, a distaste for excessive documentation and self-confidence bordering on megalomania. Within two weeks, they wrote an eighty-page paper describing proposed NT versions of hundreds of Windows APIs.

and chapter 6 mentions the NTFS spec being initially written in two weeks by Miller and one other person on Miller’s sailboat.

> Maritz decided that Miller could write a spec for NTFS, but he reserved the right to kill the file system before the actual coding of it began.

> Miller gathered some pens and pads, two weeks’ worth of provisions and prepared for a lengthy trip on his twenty-eight-foot sailboat. Miller felt that spec writing benefited from solitude, and the ocean offered plenty of it. [...] Rather than sail alone, Miller arranged with Perazzoli, who officially took care of the file team, to fly in a programmer Miller knew well. He lived in Switzerland.

> In August, Miller and his sidekick set sail for two weeks. The routine was easy: Work in the morning, talking and scratching out notes on a pad, then sail somewhere, then talk and scratch out more notes, then anchor by evening and relax.

(I’m still relatively confident that the Win32 spec was written in 1990; at the very least, Showstopper! mentions it being shown to a group of app writers on December 17 of that year.)