Since the beginning of the 20th century, American handguns have been dominated by one caliber: The .45 ACP.
In the late 80’s/early 90’s, while Glock was engineering their first .45 caliber handgun, they made the decision to pattern it off of a frame developed to try to chase the FBI contract that had been put out for a double stack 10mm. While the larger frame did allow for a significant firepower boost compared to the 1911 (the handgun synonymous with “45”), it also came with a beefier grip. The standard framed 20/21 models came with a 6″ overall–a significant step up from the 1911–and one that wasn’t a comfortable fit for everyone. The .45 model was well-received, but always took a back seat to the 9s and .40s in popularity for almost 20 years and this has been the driving force in several design changes (the Gen 4 frame is a descendant of one variation) and even the introduction of three new models to go with a new, Glock-designed .45 caliber round.
In 2007 the Joint Military Task Force announced that they were opening competition for a possible replacement to the M9 that had been in active duty since 1985… and they were looking for a .45 caliber replacement. The .45 GAP caliber had already been in production and was the first attempt to fit a .45 caliber round into a more standard size frame, but this limited capacity and just wasn’t going to be up to snuff for the new bid. A military contract would mean enough revenue that Glock took the competition seriously. It required a redesign to meet the spec sheet requirements, but rather than starting from the ground up, attempting to alter the standard 9mm frame yet again, they decided to start with the larger frame 21 and try to shave it down.
The result of this shaved down model came to be known as the 21SF and continues in production to today (although a standard Gen 4 with no grip adapters is the same size as the Gen 3 SF which makes the SF somewhat obsolete). But the early guns included a couple other features that were required by the military–a standard picatinny rail, and an ambidextrous mag release. While the Gen 4 guns have a reversible mag release, this first attempt was actually meant to be fully ambidextrous without any changes having to be made to the gun… and it turned out to not be so reliable.
After a short trial period, the military ended up deciding to scrap the project altogether and stick the Berettas for now but we got the benefit of this failure in a fairly hard to find model of Glock. It is the only model with a standard picatinny rail and it can also be identified by the finnicky mag release. If yours works, it works, but if it doesn’t then Glock replaces your frame with a new one–one without the ambi release. This model could’ve only been picked up new in 2007 and do to the problems with mags staying put, the production run didn’t last long. We do have this model to thank though, in that the Gen 4 was an improved version of what they were intending to do with the SF.