Sometime in the mid-1960's, Colt altered ALL their small "D" framed revolvers to use the same stubby grip frame.
This allowed cost cutting, since Colt no longer had to make two different frames.
After the change, the Detective Special and Cobra used grips that overlapped on the bottom, giving a full sized appearance.
The Agent continued using the tiny wood grips to give a smaller size.
Eventually, Colt discontinued the Agent, since by that time the only difference between an Agent and a Cobra, was the marking on the barrel and the type wood grips used.