Here is a very late Sterlingworth made with a flat sided frame like the SP grade.
"The FOX GUN"( A.H. Fox "Wood Grain" Catalog A.H. Fox Gun Company, Philadelphia, Pa.)
THE FOX STERLINGWORTH is a plain, staunch, trustworthy gun, at a reasonable price, that will make good all the time and under all conditions. It has all the exclusive FOX features.
The BARRELS are genuine Sterlingworth Fluid Compressed Steel-(FOX PROOF)-thoroughly tested and guaranteed for strength and shooting qualities. THE STOCK is best quality plain American Walnut, thoroughly seasoned and checkered. Full capped pistolgrip only. THE ENGRAVING consists of linework around the frame and guard. THE FOREND is a new FOX spiral spring creation, and acts as one of the greatest compensating devices ever placed on any gun. Will never break or get loose.
26, 28, 30, 32-inch barrels
Weight 6 3/4 to 8 lbs- 12 gauge
Weight 5 3/4 to 7 lbs- 16 gauge
Weight 5 1/2 to 6 3/4 lbs 20 gauge