That's interesting, Scooper. As far as Chas' split sleeve theory, I'll have to look. As far as the straight gage theory, very interesting too. The steerer on the Moto I'm working on has "Reynolds 531" stamped right on it, but still may be straight gage tubing I suppose.
BTW, the wooden plug in this steerer was quite loose, but may have been snug when installed. Wood shrinks, after all.