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The most noticeable changes to the exterior of the pistol came in 1928 with the introduction of the 1911A1 pistol.
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The changes began in 1924, with the first pistols delivered on January 23, 1924.
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the hammer spur was thinned and shortened
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The shortening of the hammer (on the M1911A1s) didn't begin until 1939. The thin spur hammer was adopted in 1944.
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the thumb safety was changed from a small thumb piece to a longer style with serrations instead of checkering.
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Colt used the short self thumb safety throughout production of the M1911A1s and used them on the Commercial Model until 1949.
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and the lengthening of the of the cut under the slide nose on the recoil spring plug housing.
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The changes on the recoil spring housing began in 1918.
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The finish of the pistol was changed from a polished blue to a grit blasted parkerized finish.
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Colt did not start Parkerizing their pistols until May 1941.
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The wooden grips were also changed to the plastic grips for improved durability and less manufacturing costs.
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Colt introduced their plastic stocks (called Coltrock) in 1940.
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The internal parts that were changed over the years were the firing pin stop
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That change took place in 1918.
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the trigger went from being machined from one piece of steel to two pieces
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The change from the long trigger to the short took place on the M1911A1 (1924), the one piece triggers continued until August of 1942 when the stamped triggers were adopted.
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the recoil spring guide was strengthened
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The recoil spring guides were "round" to the approximate style more in line with those of Springfield Armory drawings.
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the barrel was changed externally to allow faster production
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Barrels remained the same, some varied in the amount of polishing.