I'd guess that SP-/SPE-Grade demand didn't meet expectations and Savage decided to use up some frames for something that might sell. All the Flat-Sided Fox-Sterlingworths that I've seen lettered show shipping dates of May 1939 or later. Likewise the draw-bolt frame Fox-Sterlingworths that I've seen letters on all show shipping dates of 1939 or later. From my years of observations, the flat-sided Fox-Sterlingworths are in four groups, which blows that worthless serial number chronology out of the water -- 30-inch 12-gauge guns in the high 132xxx range, 28-inch guns in the high 137xxx/low 138xxx range, 26-inch 12-gauge guns in the 161xxx range, and 28-inch 16-gauge guns in the 3769xx/3770xx range. The drawbolt frame 12-gauge guns are 30-inch guns in the 133xxx range, mostly 28-inch guns in the 143xxx range and 26-inch guns in the 161xxx range.
