Hold on. There are two standard sizes for threaded headsets.

1", which has a 7/8" (22.2mm) quill diameter.
1-1/8", which has a 1" (25.4mm) quill diameter.

Granted, the first is probably your bike' size. This is SS/FG, so the bike is probably road and most road bikes with a threaded headset are 1" standard. But that's a lot assumptions. Make sure you get the right size.