[S404]

Details courtesy TooSixy

• 1993 (KD): Original USP .40 and 9mm. USP40 came out first
• 1994 (KE): Reduced the slide weight by 1.1 oz
• 1994-95 (KE-KF): Changed to a polygonal rifled barrel (Prior barrels were standard land and groove)
• 1994-95 (KE-KF): Changed trigger transfer bar
• 1994-95 (KE-KF): Changed recoil guide rod to a "captured spring version"
• 1995 (KF): Changed trigger mechanism
• 1995-96 (KF-KG): Added rubber spur to hammer
• 1995-96 (KF-KG): Changed angle on slide lock
• 2000 (AA): Added locking feature to hammer strut support (lanyard safety)
• 2001 (AB): Converted captive recoil spring retainer from c-clip to machined end on USP Compacts (not USPf)
• 2005 (AF): Redesigned firing pin, firing pin block, and firing pinblock spring

This is great information.