S.F. stands for stands Speed Faster, the old name for this line of Swingline Staples.
S.F. 39 staples are for older, discontinued heavy-duty staplers Swingline Staplers 415, 900xd 390nx. They are made of thicker wire and have shorter points than the newer S.F. 13 staplers. S.F. 13 staples are specifically designed for current lineup of manual Swingline Heavy-Duty Staplers.
Swingline does not recommend using S.F 13 or S.F. 39 staples in staplers from other manufacturers. Some staplers are designed to operate with staples make of thicker or thinner wires. If S.F. staples are too thin for the stapler, they won't completely discharge or pierce the whole stack. If S.F. staples are too thick, they will cause your stapler to jam.
Incidentally, S.F. 1, 3 and 4 Staples are for designed for basic 20-sheet capacity staplers. S.F. 3 and 4 staples will work great in most standard staplers. However, S.F. 1 staples are only recommended for older Swingline Staplers due to the relatively thick wire and short points.