Hal, Security is adjustable sights, Speed is fixed sights. Actually kind of makes sense when you think about it... of course, the naming of the "Six" series was a long time before the idiocy of Vaquero/New Vaquero for two otherwise different guns.
If Ruger made a "six" series revolver in 9mm, I don't remember it, so it was either a very short production life or not a main catalogue item. (now, in a few minutes, somebody will pop up with a pic, and catalogue #, dates of production and numbers, just to prove me wrong...again...
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The main thing is that 9mm Luger is considered a personal defense round, as its primary utility. There are much better rounds for hunting, and even better ones for target use.
SO that means to have any real market it has to be a DA revolver. And rimless rounds in DA revolvers are..complicated. Various DA guns have been made in 9mm, but never seem to stay in production. In terms of numbers, the most common 9mm revolver you will find is the Ruger Blackhawk .357 convertible. But its not sold as a defense gun, nor as a 9mm, its a single action .357, with an extra cylinder for 9mm Luger.
The only real practical advantage (or use) for a 9mm revolver is if you can only get (or use) 9mm ammo.
Now that I think back on it (without looking anything up) I do seem to vaguely recall there was a "Six" in 9mm. I think most of them were sold outside the US, or intended to be. Certainly not often seen today...