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 M-1900 were equipped with rear milled slide grooves but then 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. Total production of the M-1900 was 4,274.