Ruger Only loads...
When Ruger introduced their .45 Colt Blackhawk on the .44 Mag frame, handloaders began experimenting with heavier than factory standard loads.
They found that the big frame Ruger .45 Colt would handle loads in the .44 Mag class. These loads were written about in handloading magazines, gun magazines, Gun Digest, etc. They became known as "Ruger Only" loads.
They were only in .45 Colt caliber, never any other. And, at the time, there was only ONE Ruger in .45 Colt.
Today they are still called "Ruger Only" loads, BUT Ruger today also makes .45 Colts that are NOT SUITED for the heavy Ruger Only loads. This is a subject that needs careful examination, and carefully matching the load to the gun. The large (44) frame Blackhawk & New Model Blackhawk will take heavier than factory standard loads. The VAQUERO will also (same frame size)
The NEW VAQUERO will not. Note the slight but IMPORTANT difference in the names Vaquero and New Vaquero. The guns are marked (left side, below the cylinder window). If your gun says New Vaquero, do NOT use Ruger Only loads!!!
I do not know about the "flattop" frames, I don't own any. Perhaps someone who does can comment on where they fall in the strength class.
Ruger has a confusing number of frame sizes and names that sound identical or very similar, and if you are going to go beyond factory load levels its rather important you use caution and do some research to avoid problems.