The original Ruger Blackhawk, also known as the flat-top, was produced from 1955 - 1962. The .357 was built on a medium, Colt SAA-sized frame. The .44Mag flat-top was built on a larger frame. Both used the same XR3 grip frame, which is a one-piece dead-ringer for the Colt SAA/Navy pattern. Both had steel ejector housings and both had the all steel Micro rear sight. In response to Charger, there are ZERO strength issues with the original .44 flat-top. The Super Blackhawk was introduced in 1959 and with it came the protective ears around the new proprietary aluminum rear sight, unfluted cylinder and longer steel Dragoon-style grip frame. The receiver is basically the same. These guns were phased out in 1962 when the .357 sized frame also received the protective sight ears, the aluminum rear sight and "improved" XR3-RED grip frame and aluminum ejector housing. The .44 flat-top was simply discontinued and the Super remained as its replacement. Again the .44 flat-top and Super Blackhawk receivers are the same size and strength.
Due to consumer pressure, Ruger brought back the Colt-sized medium flat-top frame in the form of the 50th anniversary .357 Blackhawk and New Vaquero in 2005. Except now the XR3 grip frame is steel, as is everything else. In 2006 they introduced a new 50th anniversary .44Mag flat-top built on the same large frame as before, with the new steel XR3 grip frame.
The 50th anniversary .357 platform was eventually re-utilized for the .44Spl and .45 convertibles. These began as Lipsey's exclusives and the standard Blackhawk configuration became a regular catalog item. Now there are stainless Blackhawks and Bisley versions as well. A group known as The Shootists, had Ruger build a set of 7½" Bisley flat-top .44Spl's with a bright polished blued finish and special serial numbers. The Lipsey's exclusive .45 convertibles are the very first Ruger .45Colt built on the mid-frame Blackhawk. They are not safe for "Ruger only" loads but may be loaded to 22,000psi.
There was also a run of .41Mag flat-tops identical to the 50th anniversary .44's.