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.
Well, your explanation makes sense, but then I was misled by Ruger when I originally contacted them about being able to shot the highest powered .45 LC ammo when I first got the gun (~2014). Buffalo Bore (who makes this very high powered .45LC ammo) does call the Ruger Flattop .45 as a 'medium' sized frame, and not to shoot their real high powered ammo in it, but Ruger said otherwise. Of course, Ruger's answer did not explain anything about why I should not shoot that ammo, and did say that there was only one frame for the .45LC Blackhawk. Of course, it is possible I misunderstood them, but I was quite explicit, so I do not think so. I suspect that whomever answered me was not aware of there being two different size frames for the Ruger .45 Colt, nor the implications.
My gun is from Lipsys, and that might be part of the problem with 'faulty' info from Ruger. I am sure glad Buffalo Bore had those disclaimers on the .45LC ammo pages. Of course, I have no way of examining any differences since I own only the one Ruger .45 Colt, and it is the Flattop' Convertible. By the way, I do love the convertible capability so I can shoot 45ACP as well. So, thanks for your info and response.
Do you know if there is a full sized frame Ruger .45 Colt that is also a convertible and that can also shoot .45ACP ammo with a cylinder swap?