Just a bit of history.

In my earlier posting, I said the Trooper and the Highway Patrolman were closer to each other; by that, I meant both beefy but the HP is an N frame and is larger than the Trooper.

In the 1970s, S&W made up some prototype L-frame 586, complete with heavy underlug on the barrel, with an eye to taking on the Python head to head. But S&W reportedly backed off since in those days the competitors were "gentlemen" and didn't copy each other in styling.
But by 1980, S&W changed its mind and hence, the birth of the 586/686.
Again, as I said earlier, the 586 was designed to fit Python leather and could use Python speedloaders, all a way of making it easy for police to drop the Python-sized revolvers (sometimes called .41-frame size).

Today, the Ruger GP 100 also takes the 586/Python speedloader and will generally be a good fit in either gun's leather.