This Colt 1908 Experimental Prototype was estimated to be made in 1953 and was essentially the same gun, but was adjusted slightly for weight and concealment. Its slide was shortened by 1/2" and it's frame was made of a much lighter metal alloy. All three of these prototypes also have the much more user friendly 1911 style magazine release, which definitely makes magazine changing a lot simpler. Thus ended the relatively short but highly successful model M.
It started in 1903 and continued through the 2nd world war. With few surplus military contract guns being sold commercially in the late 1940's. During that time approx. 570k pistols were produced in .32 and 137k in .380. Lastly there were these 3 prototypes in the 1950's that were the official end for the Model M.
The factory letter reads: "Significant redesign of the Model 1908 Hammerless Model M. Shorter barrel and slide. Profile of grip frame changed noticeably by addition of contour on front grip strap, altered back trap profile, as well as altered grip safety, slide stop and slide lock safety. Base of magazine marked "Cal.380.Colt" Frame and magazine of altered configuration at butt. Altered slide, prototype configuration with blue still fixed and open sights. Compared with modified M Model 1908 pistol serial Number 1378?7. Subject pistol kept by the Colt factory as an experimental prototype of a new design of the model M Pocket 1908 pistol.
Further records show that this prototype GX2501-2 was sent to the Arms Engineering department on 2/2/53.