1) A moonclip originally allowed a revolver to shoot a rimless semi-auto cartridge. The clip holds the cartridges together in the pattern of the chambers and the entire assembly is inserted in the cylinder. Today revolvers using rimmed revolver carridges are being modified to use moonclips. Moonclips come in full (holds 6 cartridges), half (3 cartridges), and 3rds (2 cartridges).
2) Adjustable sights are great if you plan on using several different loads. All loads don't shoot to the same POI (point of impact) even though you aim to the same point (POA). So no matter what you shoot you can adjust the sights so your rounds will hit where you are aiming.
No you can't add adjustable sights to a fix sighted revolver without modifying the revolver.
3) S&W numbering system does have some glitches that causes confusion. Before the 3 digits were used the adjustable sighted .41 magnum was the Model 57 in carbon steel (blued and nickeled). When stainless steel was used the number "6" denoted that so a ss .41 magnum became the Model 657. The new lightweight alloys are denoted by a "3" so an adjustable sighted lightweight .41 magnum is the Model 357. The PD represents "Personal Defense".
Now as the "6" represents stainless steel the number "5" represents carbon steel which blued guns are made of. The Model 520 is the blued steel adjustable sighted L-frame .357 magnum. It replaces the older Model 19. It doesn't make sense because you would think it would be the Model 519 and not the 520, but for some reason S&W decided to make the ss versions as the Model 619 fix sighted version replacing the Model 65 and the Model 620 as the adjustable sighted version replacing the Model 66. Clear as mud ain't it.
Keep reading the catalogue and then read about all the models made in the last 50 years and a lot of the numbering system may have you totally confused. When they made the first stainless steel Chiefs Special, the Model 60, the numbering system which did make sense till then went out the window.
And you thought you were confused before.
