It was the "Smython" that led to the Smith L-Frame revolvers.
Well the K frame magnums had failures which resulted in a redesign of the K frame magnum line. I don't think S&W cared about Smythons really. The forcing cone cracks and complaints about the K frame mags 13/65 and 19/66 being too light, therefor amplifying recoil and possibly being shot loose were what caused the L frame change. Some people theorize it was because of the Colt Python, but that design was nearing the end of production when L frames came out. S&W simply found out that they needed to make a full forcing cone medium frame magnum with more heft. Also, regarding how they added weight by making the lug full instead of half, if you want to keep a frame size the same, but add weight at the same time, there are not many ways to do it.