Complex sentence: The Cobra is the same overall size and configuration as the famous Colt Detective Special and uses the same size "D" frame, except that Cobra's frame is constructed of lightweight aluminum alloy as compared to the all-steel frame of the Detective Special.
Split into simpler sentences: The Cobra is the same overall size and configuration as the famous Colt Detective Special and uses the same size "D" frame. However, the Cobra's frame is constructed of lightweight aluminum alloy as compared to the all-steel frame of the Detective Special.