This is a unique documented Colt Model 1900 prototype Browning Slide Lock prototype pistol built on a Colt Model 1900 Sight Safety semi-automatic pistol. There are 5 important patents issued to Colts during the evolution of the design of the 1911 Government Model pistol. The first prototype demonstrates the “Parallel Ruler” hesitation locking feature which made possible the use of powerful pistol cartridges in this and future Colts Government Model pistols. That hesitation lock prototype is in a Browning family collection in Ogden, Utah. This Slide Lock Prototype features the slide lock feature allowing faster reloading and now a feature of almost all semi automatic pistols. It was the prototype for the second patent of the five. The remaining three patent prototypes for the single link barrel, grip safety, thumb safety and other features are now in private collections. John Browning developed and engineered this fix specifically to resolve one of the weaknesses identified during the U.S. Army test of the Model 1900 pistol. Testers remarked that it took two hands to get back into action after the last shot was fired and asked for a design change to allow for faster reloading. Using this pistol, serial number 1433, as a base John Browning personally made the modifications which backed up the patent application and served as the model for production adoption of the improvement. This specific pistol is photographed and described on page 66, 67 of the book “The Government Models” by W.H.D. Goddard. As noted it still retains the rear sight/safety with the left side of the slide marked with the two line/two block marking of: “BROWNING’S PATENT”/PAT’D APRIL 20. 1897” followed by “COLT’S PATENT FIRE ARMS MFG. CO./HARTFORD. CONN. U.S.A.” with the circled Rampant Colt trademark behind the serrations. The right side of the slide is marked: ‘AUTOMATIC COLT/CALIBRE 38 RIMLESS SMOKELESS” ahead of the ejection port. The left side of the frame has the serial number “1433” with a small hole directly behind it with the Browning handmade prototype slide stop as pictured in the above noted reference. This pistol has the high polish Colt commercial blue finish with niter blue barrel and trigger, casehardened rear sight and hammer with smooth walnut grips. This pistol still retains its original all blue and unmarked prototype magazine that has the small square notch cut on the left front edge of the magazine that is designed to allow the magazine follower to actuate the slide stop lever. Provenance: Shipped to Browning Bros. January 4, 1901, Colts Factory Collection, Collection of The Connecticut State Library, Collection of William H. D. Goddard.