Thanks to Legacy Collectibles, today we have a unique Colt prototype pistol to look at today - a double-action variation of the 1911. Judging by the grips and markings, this was made in the late 1950s - but I have been unable to find any documented record giving details. The double action capability was added by dropping the trigger mechanism down a bit, extending the trigger, and adding a linkage between lugs added to the side of the trigger and hammer. The original single action sear mechanism and trigger bar were left intact, and the added mechanism is simply a cocking lever - firing is done by the original elements.

In addition to the D/A linkage, another safety mechanism has been added to the slide. What looks like a decocting lever is actually a simple firing pin retractor. When engaged, it pulls the firing pin forward such that the hammer does not strike it when dropped. This effectively safes the gun. Presumably, this would have been a replacement for one of the other safeties already present on the design (grip and thumb), as it seems rather redundant when added to those others.