That's a really odd I.D. for a 25.4mm O.D. steerer.

25.4mm - 18.0mm = 7.4mm/2 = 3.7mm wall thickness for the steerer tube. For the past 70 years or so, 25.4 O.D. steerer tubes have typically had 1.5mm walls and took 22.2mm quill stems. Low tensile strength steel hi-ten steerer tubes typically took 21.1mm quill stems.

Freddie Grubb made high end frames. Are you sure you measured the steer I.D. accurately?