Wow ok, I've never seen that in the wild. RCDs are required on all circuits including lighting in the current regs (i.e. old installations can not have them and be compliant, but they have been required and socket circuits for a long time; bathroom & outdoor lighting for less; all lighting more recently). Maybe these sockets are for when you have no more room in the panel and don't want to replace it, but need it upgraded due to other work or to pass an EICR to let the property? Not sure.
The university halls that I lived in had those RCD sockets installed. The halls dated back to the early 60s - the sockets + Ethernet were installed in perimeter trunking. I suspect it was the cheapest option available to the university to modernise the electrical infrastructure inside the halls.
It's weird on imported goods here in the UK too. Macs are infamous for the fuzzy feeling of the double not-quite-insulated chassis. My Marantz amplifier (not US made but also sold there) that arrived today is the same. Weird looking plug too, like they had to go out of their way to source a dummy Earth to Earthless kettle lead and the result is bizarre and cheap looking but it's all they could get, because it's a weird requirement.
Grounded outlets became code for kitchens and baths in 1961, although it wasn't that uncommon to see in '50s new construction. 1971 code started requiring them throughout the house.
Example US looking one I was seeing: https://i5.walmartimages.com/asr/e9c29ec0-5eeb-40c3-8cde-e2f...
I suppose Earthed sockets are less common there anyway right, so that limits how common these could possibly be.