Detachable shoulder stocks for various handguns go back to at least the Civil War, if not before. I have even seen caplock single shot pistols having them. There was even experimentation during WWII with shoulder stocks for the Colt M1911 using a modified hammer spring housing for attachment, converting it into a carbine. But in the end, it's not a particularly worthwhile idea as it is cumbersome to carry around a stock and it takes time to attach it when you need it. Probably the only handgun to use a shoulder stock with any degree of success was the Mauser C96 and its variations. As the Chinese seemed to be very fond of the C96, I'd guess that they may have adopted some other handguns laying around, such as this one, to use shoulder stocks. But I don't believe S&W ever made any of their solid-frame revolvers to use shoulder stocks.

I never quite understood why handguns with shoulder stocks were outlawed (without a permit) under the NFA. But many things were outlawed under NFA that I don't understand the reasoning for.