The SP-/SPE-Grade was introduced by Savage in their June 12, 1932, Fox wholesale and retail price lists. This magazine ad was a bit optimistic as to price, --

as the price with the non-selective single trigger on the June 12, 1932, price lists was $54.
Early on the SP-/SPE-Grades were perfectly plain and a bit cheaper than the A-/AE-Grades but by the March 1, 1934, price lists they were the same price. Also, in the March 1, 1934, wholesale price list they added a skeet model --

About this time the SP-/SPE-Grades got a bit of line border engraving and some stippling on the top of the breech. For 1935, a fancier skeet gun, the SP-/SPE-Grade Skeet & Upland Game Gun with quite a bit of coarse scroll engraving was introduced --
The SP-/SPE-Grade Skeet & Upland Game Gun remained through Savage's 1939 Fox catalog.
All of the SP-framed Fox-Sterlingworths I know of that have been lettered show ship dates of May 1939 and later.