SAS mentioned in the London Sourcebook (way back in first Edition), also cited in the 3rd Edition follow-up Shadows of Europe but there aren't any stats or history details, etc.
I know that there was a chapter on mercenary outfits in obne of the third Edition sourcebooks, State of the Art 2063 I think. Met 2000, Tsunami, 10,000 Daggers etc. That goes into the background and fluff along with new toys and rules. The only other book really going to town on military/mercenaries in the Sixth World was the second Edition Fields of Fire sourcebook, now available as an ebook download.