Okay, I guess I should have done a google search on this topic before posting. Anyway, here is a cut & paste from the sight1911.com web page on USAF OSI:

The last military modification of the M1911A1 came in the early 1980s. The United States Air Force wanted to issue its Office of Special Investigations (OSI) personnel something a little potent, but nearly as compact as a .38-caliber snub-nosed revolver. Starting with the basic M1911A1 acquired from the U.S. Navy as surplus, unserviceable weapons, the slide and barrel were shortened by .75"(1.905 cm). The frame was also shortened nearly .5"(1.27 cm), requiring a special six round magazine to be created, with a finger rest to create a better gripping surface for personnel with large hands. The front edge of the triggerguard was squared and the frontstrap stippled for better gripping. A ramp-type front sight was installed on the shortened slide and an ambidextrous safety was built and added to the pistols. Costing U.S. taxpayers less than $100 to rebuild, only a small number of these OSI pistols were built.