http://www.leviton.com/OA_HTML/ibeCCtpItmDspRte.jsp?sitex=10026:22372:US&item=4206
Far out
I don’t see what the T slots are good for. The description states 15A, 120V.
Really old 110V stuff had two sideways prongs.
That would be old 20 Amp stuff that would blow a normal 15 Amp fuse. Somehow the Leviton 15 A rating has to be a typo. 
Or they got it wrong at the plant in China.
When I look at 406.7 it really looks to me like the 2-20 was a 240v plug
Marc, you’re just a young feller, so you can’t be expected to know this. I don’t even know if this is right myself.
Two flat prongs would be 220V. One flat, one upright would be 110V, 20 A, so you couldn’t accidently plug the low voltage device into the 220 outlet, right?
Now if you want to produce a Polarized receptacle, like the Leviton unit in Greg’s picture, why would you give it double t-slots? Polarized 20A would only want one t-slot for each plug-in. Like the red one there. 
You guys need to throw out everything you think you know about modern plug configurations out the window. Again I state, really old 110V stuff had two sideways prongs. How do I know? I work in really old houses every day.
Marc,
What kind of appliance did you see have the two prong tandem plug on? I know I have seen many of the old two prong T slot receptacles, but I guess no one except antique collectors have appliances with those plugs?
I thought I had an old Hubbell two prong plug laying around with one prong tandem and one prong straight, thing was labeled 15 A 125 V. The older stuff WAS weird. :o
Another wierd thing is that the old T slot receptacles had dual ratings, something like 15 A 125 V and 10 A 250 V. This modern remake does not. Also note that it says NON UL. So perhaps the only real purpose for a receptacle like this would be in another country (maybe in south America or Mexico.) Where there isn’t as stingent as listing standards. Just my guesses.
Floor lamps and console radio’s come to mind right away.
Thanks Marc, we don’t see that kind of receptacle hooked to 110 in my area, but I don’t doubt that you’ve seen them in you neck of the woods, put in before standards were established by some stubborn …? 
Anyway, the modern 6-15R and 6-20R receptacles are rated at 250 VAC, and look like this pic.
Typically with that kind of repectale it is used on older two wire circuit sans ground wire.
I ran into like that pretty often with some olderhome like that arrangement and alot of " handy peoples " make a common mistake with replaceing the receptale and don’t relized that you can have two choice one is have GFCI at first receptale before downstream of that circuit and labled " NO equiment grounding " or stay with modern receptale without grounding slot verison.
Merci,Marc