A variant of the Glock 21, the Glock 21SF hit the market in Spring 2007, in an effort to make the fullsize .45 Glock more appealing to shooters with smaller hands. The Glock model 21SF, stands for "Slim Frame".
The Glock 21SF was originally developed and designed to compete in the currently cancelled US Military Trials that were seeking a .45 ACP pistol to replace the current Beretta M9. The first Glock 21SF's developed and designed for the military included popular revisions such as the picatinny rail system (which replaces the common 1913 rail system on most other Glock pistols) and the ambidextrous magazine release, along with the well designed frame reduction designed to ensure that the grip is more universal. The Glock 21 grip reduction extracted material mainly from the backstrap of the grip.
Initial statements suggest that only a small number of Glock 21SF's in full military spec will infuse the public marketplace, and that Glock is going to mass produce a grip reduced model for mass distribution with the 1913 style frame rails.