In the early 1950's Colt was preparing to introduce a modified version of the Model M. We know this because of the existence of several post-war prototypes. It is a rough shop prototype made in 1951 from a scrapped military contract .380 frame.
Because it was made of rough unfinished steel it is easy to see the structural changes the post-war Colt engineers made to existing design. Notice where the angle of the grip has been changed by cutting and welding at the back-strap and again at the bottom of the magazine well. A pre-war Model M magazine was also cut and welded to accommodate the new shape.
The slide for this piece was completely redesigned and machined from fresh raw steel. It was never fitted with sights and no barrel lug groves were ever cut into it's interior. Therefore, it is assumed that no barrel was ever made for it either. It can also be assumed that this was just a rough shop prototype whose sole purpose was to establish the basic size and shape the engineers were looking for.
The factory letter states this pistol was finished "In-the-white" except for the blue slide stop. Special features noted on the letter: "Significant redesign of the Model 1908 Hammerless Model M. The length estimated at 3 5/8 inches. Missing a few parts. Profile of grip frame changed noticeably by addition of steel on front grips trap, on grip safety, and on back strap, as well as magazine at butt. Altered slide, prototype slide incomplete and not fitted with sights. The accompanying Engineering department tag dated "11/16/51" and initialed "TES" by Colt Factory Engineer T.E. Sturdevant. Tag also indicating "original Modified "M" / Standard gun rebuilt to/try out various new features..... 24 1/2 oz. weight."