First developed by Colt as the Model 1971, with approximately 30-50 being manufactured, this model would become the Stainless Steel Pistol (SSP) and was conceived as a way to get the jump on a future U.S. military handgun request. Before any formal requirements had been fielded, the company had managed to develop a solid read on the then-current trends in firearms, particularly the rise of what would later be dubbed the "Wonder Nine", the 9mm double-stack double-action semi-automatic pistol. While the knockdown force of the 45 ACP had (and still has) its adherents, the appeal of being able to standardize to the 9x19mm NATO cartridge and get a 13+ round magazine capacity in the process was rather broad. Colt's read on the Wonder Nine was a radical departure from the 1911A1, completely changing much of the internal configuration and external appearance. The slide is secured with a set of full length external frame rails, and the barrel via a link-free ramp assembly, while the hammer and mainspring are configured as a one-piece "en block" assembly that can be slid out of the frame for maintenance and repair. The SSP had an additional feature, specifically that it was designed to be convertible between 9x19 NATO and 45 ACP with just a few parts. While this option was not part of the later XM9/XM10 trial requirements, the post-XM9 trials for a SOCOM-particular 45 handgun and the recent American adoption of the caliber convertible Sig-Sauer M17/M18 to replace the Beretta M9 (which beat out the SSP and others at the XM9/XM10 trials) makes this seem like a nearly prescient move. Following the trials no commercial production was pursued, and the SSPs made for the trials were either tested to death, given to VIPs as gifts, or disappeared into the American logistics system, and few can be found today. This SSP is marked "COLT'S MODEL S S P/CALIBER 45 ACP" on the slide, with "45 ACP" on the chamber and the one line Colt address on the right side of the frame over the "GX 9730" serial. Sights are blade front and square notch rear, with the rear dovetailed into place and the front mounted on a small detachable panel at the front of the slide. Mechanical features include an ambidextrous pin-blocking safety, right handed slide catch and magazine release, smooth trigger, serrated grip straps. Magazine absent. The included factory letter confirms it's configuration and further states "that this firearm was kept by the company in the Engineering Vault. Items from this collection were used by the company for test, experimental, research & development, and for other related purposes". Provenance: The Colt Engineering Vault