Your fork might have been on a Schwinn road bike from the '70's or '80's, as many of these bikes had Tange forks and an odd sized inside diameter of the steerer and stem quill (of course, Tange made a LOT of standard-sized 1" threaded forks, also). The inside diameter of the steerer tube would be 21.15 mm, or .833" (usually stamped .833" on the stem), instead of the usual 22.2mm on a 1" threaded fork. As Sheldon says, it's not JIS, but as far as I know on road bikes of this era, they're pretty much "Schwinn specific." The good news is that there are lots of old Schwinns from this time period still out there, and if you look hard enough you can probably find a stem that will fit on ebay. The most common brand of stem used on these bikes were the Japanese-made SR's (LOTS of standard-sized SR's out there, too), and I was able to find one recently on ebay by looking for a distinctive ridge near the top of the quill, where the stem was necked down to the smaller size during the machining process. I kind of think all the stems started out the same during manufacturing, and SR simply machined the Schwinn-specific size down a little more during the manufacturing process. Regardless, the ridge I'm talking about is clearly visible on one of these .833" quill stems, and I've yet to see one that isn't stamped .833" on the lower part of the quill.........if it turns out your fork's not from an old Schwinn of this era, then I have no idea, and disregard everything I've said.
Good luck-
