Wrong again vlavalle. Rclark has it right. The NM .45 Colt Blackhawk Flattop(look at the roll mark on the left side of your Flattop convertible and you will see Blackhawk) is made on the medium sized frame. Because of this the cylinder window is smaller which means the cylinder is smaller. Due to this the cylinder walls on the NM .45 Colt Blackhawk Flattop are thinner. (The.45 Colt is a big cartridge. In order to fit it in the smaller cylinder, the cylinder walls are thinner.) It has nothing to do with the top strap. The only difference in the design of the frames (other than over all size) are the ears for sight protection. Some folks have even been known to machine off the ears to make a large frame Flattop. Do a search on here and you'll find this has been discussed before on this forum and the cylinders and frames have been measured with machinists calipers for both cylinder walls and overall cylinder size. If I recall correctly the OM Blackhawk flattops were built on the large frame. It was in these guns that the Ruger Only loads were developed.