It is possible that a 1.125" headset was used but only if the head tube was about 34.0mm inside diameter. A 1" steerer fork could be installed with adaptors/shims. But this would not be the usual or common at all. The fork crown race would need to be shimmed or a specific one with the bearing dimensions of the 1.125' headset and the crown seat diameter of a 1" headset (26.4mm or 27.0mm).

Are you sure the shim wasn't inside the stem and not overlapping with the upper bearing race (the one with the conical compression ring fitting in it)? The use of a 1.125" stem on a 1" fork is FAR more common then mixing the fork and head tube specs.

Luckily all this is moot soon enough with a few simple measurements. here's a dimensional chart to discern the different standards. Sheldon Brown's Bicycle Headset Dimension Crib Sheet Andy.