The Colt Improved Model M1911 is primary referred to as a "Transitional Model". The process to improve the M1911 started in 1920. The ordnance department proposed to shorten the hammer to eliminate hand pinching, extend the grip safety, create a bump on the mainspring housing, cut back and knurl the trigger face and mill finger clearance cuts on the sides of the receiver behind the trigger. The model designation wouldn't change until 3 years later.
On June 12th 1923 Springfield Armory awarded a contract to Colt for the manufacture of 10,000 Improved Automatic Pistols. They were assigned Serial numbers 700001 - 710000.
All transitional models have the inspectors mark of WTG for Walter T. Gordon, Captain. Roughly the first 2,000 pistols used a different stamp than the last 8,000 pistols. Note the inspectors mark created a broken circle caused by the initial G. The later pistols will be enclosed within a double circle.
Barrels in Transitional models are a bit different when compared to the 1918/1919 barrels. There is an HP on top of the barrel that share a common leg. The majority of transitional barrels will be marked with a K marked on the underside forward the lug.