The Smith Model 59.
It's a logical evolution of the M39 Smith. As I recall, the M39 came out in 1955 or '56. The M59 was in the middle '60s, I think. (They never did impress me, nor do they now.)
Part of the evolution must mention the Walther P-38, the first widely accepted double-single action pistol with decocker, and the Browning HP, being the first widely accepted double stack magazine. Those guns came out in 1938 and 1935, respectively. However, neither fit the full defination of "wondernine".
The CZ 75 came in 1975, didn't it? Somewhere around there. The 92 Beretta was introduced in the '80s. The 92 Beretta, like the M59 Smith, was an "updated" version of an earlier Beretta, the model 1951 Brigadier. Currently seen in the "Helwan" incarnation. Fair pistol, rotten safety... but I digress.
The term "Wondernine" I first recall from the middle to late '70s. I think "Guns and Ammo" magazine was where I first saw it used, but it may have been coined elsewhere.
Being an old 45 shooter and considering my 357s as my "small" guns, I've always wondered about nines!