Shameless, I do believe that SF stands for "Slim Frame" ok.

Now looking at the GLOCK model book, I do see a Glock 20 and 20C. The 20 C stands for compact! The Slide Length of the regular 20 is 7.59 inches or 193mm. Listed in the book.

The also make a GLOCK 29 with a Slide Length of 6.77 inches or 172mm. in the 10mm caliber.

Thank Our Veterans!
GOD Bless Them All

UNIONS BUILDING AMERICA, SALUTE ALL THE UNION TRADESMAN