Just a few words to clarify why an L-frame was developed. There was nothing wrong with the original K-frame .357 Magnums - with SAAMI-spec'd ammo in heavier bullet weights (140-158 gr). As you go lighter, of course, the velocity increases. This made 110-125 gr .357 Magnum bullets require 'hot' propellants - producing more hot gases escaping at the b/c gap - and possibly eroding the smallish forcing cone. While cracked forcing cones undoubtedly occurred, they can be fixed by a barrel change. This likely would never occur in normal use, as it was estimated as taking 20k lite/hot loads minimum to cause this damage. There are millions of 13, 19, 65, & 66 models, all K-frames, still in use - and great .357 Magnums, too.
The L-frame has a front frame piece that is .040" thicker where the barrel mounts. This permits a .025" larger OD on the barrel - and more 'meat' at the .357 Magnum's forcing cone. They also gave it a taller cylinder opening - allowing a 7-shot cylinder to be fitted. This also permitted a 5-shot .44 Special variant to fit the frame, albeit with a really teeny forcing cone (No Keith-level loads for these .44 Specials!). The gripframe of the L-frame is the same size as the K-frame, thus, they take the same sized grips. Several years ago, S&W dropped the last of the K-frame .357 Magnums, the 65 & 66, and replaced them with 7-shooter 619 & 620 models. As the other .357 Magnum L-frames were full-lugged, they appeased we traditionalists with the 619/620 models and retained the partial lugs of the 65/66 models they replaced. Gripped similarly, a 4" 66 and 620 aren't too different... great 'pointers'.
The K-frame won't go away... the original M&P's latest variant, the 10, may always be in the S&W line - as is it's SS sibling, the 64 - and the adjustable sighted version, the 67. Then there is my latest new one - a 617 from last fall. That's a ten-shooter in .22 LR.
You can have fun in one caliber - .38 S&W Special - with each frame size. A J-framed 642 as a five-shot pocket protector, a 4" 64 as a K-frame plinker/house gun, a 5" half-lug 686+ as a fun seven-shooter plinker, and a 4" 627 Pro eight shooting, N-framed, super-fun plinker. Yeah, I won't ever bring them all to the range at the same time again - the indecision as to which to shoot was maddening! Also, I shot up a ton of my 125gr JHP .38's.
Stainz