Good eyes, Hemingray! Yes, it is a "T-slot duplex"- which ought to give an idea of how old this install is.
As for the use of the "3-0" box....well, these boxes were once almost universally used where things were surface-mounted. Our current "$ square" didn't come into vogue until the '60 (if I recall correctly).
#121452 - 07/18/0508:32 PMRe: Use in Fair Weather Only?
The install is fully exposed to both the weather, and alley traffic. (I was standing on the pavement when I took the pic). Maybe next winter I'll take a pic of this box with an inch or two of snow atop it. In all fairness, it IS possible that the receptacle is 'hot' only when the outside light is on (a common arrangement here).
As for the condition of the materials, well, you have to remember that this is Reno, "high desert." Our snow and rain don't last all that long, and the thinner air leads to very little rusting of even bare metal. Another clue as to the age of the install is the "indent" style EMT connector; that is, no set screw or compression nut.