Actually, I've seen this before.
The old ("non grounded" / "polarized") outlets (pics 1-3) quite often had single terminals on them, back in "the day" (pre 1940's). I've pulled a bunch of them out of houses around here. IMHO (best guess) they weren't big on splitting the outlets back then, and didn't think that the extra molding of the Bakelite was worth it.
Of course, maybe the increase in two-wire circuits forced the industry to accept the two screw as "standard", once more developers / EC's demanded it.
