NEMA 1-15R receptacle, legal for new builds until 1968, met code until 1972.
HAHA, that's sure a weird looking version of a NEMA 1-15... Usually the two prongs are centered vertically instead of being based on a back housing that could support the ground prong.
Makes sense, though, since production must be extremely limited on those by now.
Those are still available here for replacement purposes where a circuit is supplied by a legacy 2-wire, un-grounded cable and it is impractical to fish a ground wire to the box. They are still valid and allowed, just not for new installations. Most of the time though, for retrofit people go with a GFCI with the ground left open, which last time I checked that specifically, was still allowed as long as you put the little stickers on any downstream plugs that you put a 3-prong plug on which doesn't actually have the ground connected. It is certainly sub-optimal, and you lose any shielding effect for devices with a grounded chassis, but it is allowed.
I could go over to Home Depot right now and buy one.
(They're like $10 each instead of the $0.50 for a regular receptacle, but they're still there on the shelf.
)
EDIT:As for earthing. Glass houses and all that.
I was talking about the whole system being above ground due to some fault.
We still use plenty of two-prong extension cords and devices over here. Your disco-lights or music system from the example might not even have a ground, though if it's professional level gear it will, and then, yeah, you're back to grounds which aren't actually grounds or minor differences between panels causing ground loops, etc. Welcome to being a pro sound and lighting engineer, we fight that crap all the time on gigs. But that has nothing to do with three phase vs single phase. The same issue exists either way.
Most small things around here don't require the actual ground, but your neutral sure should still be at ground potential.