The most prominent feature of the Model 1900 was the sight safety, which while a good idea in theory, did not work out well in execution, and proved to be unpopular. When pushed down the safety blocked the firing pin, when pushed up it performed as the rear sight. This gave an immediate visual indicator of whether the safety was on or off when one went to aim the pistol, without resorting to feeling for the safety lever. The first 1900s were equipped with rear milled slide grooves but when it was found that this could interfere with the sight safety (one could inadvertently flip the safety on or off while manipulating the slide) the milled slide grooves were moved to the front. This safety was used in about the first 3,000 production pistols. Colt then installed a conventional rear sight and eliminated the sight safety although the cut in the slide remained but was plugged. The grooves, remained in the front of the slide. There are earlier sight safety M1900s that were factory retrofitted with the new fixed sight. Although Colt considered a new firing pin safety mounted on the left side of the slide with one prototype, it did not produce such a model. This was possibly a mistake as the lack of safety features probably affected sales – the recommended way to carry a safety-less Colt automatic was with an empty chamber, requiring the user the rack the slide to chamber a round before firing – which made it slower to get in action and required two hands, as well as making it impossible to render the weapon safe after firing a partial magazine, without dropping the magazine and extracting the chambered round, itself a dangerous operation in a gun without a safety. This was a major complaint of the military when they tested it, something Colt was for some reason loath to fix in the 1900-1902-1903 series of pistols. The only exception was the new small-sized Colt 1903 Pocket Hammerless, a completely new design with a grip and frame thumb safety at the start of production in 1904. It is possible that Colt found it cheaper to introduce the features into a new model, and the sales of the 1903 Pocket Hammerless were spectacular in comparison to the 1900/1902/1903 large frame automatics. By contrast, Mauser C96's and Lugers had manual thumb safeties from the outset.