TLDR: Wrecked by series arcing, from inserting in loose socket. The pins are scrap now. Don't use AC sockets that are loose.
This is due to series arcing: power was flowing through the adapter normally, but making poor contact. AC mains voltage electricity was able to leap across the very small gap between pin and socket. This arcing acted exactly like arc welding (was arc welding) and gouged and pitted the metal, and made smoke/soot (probably from the poor grade of plastic of the Chinese socket). This worsened the contact and worsened the arcing. Arcing like this makes an impressive amount of heat - which dips the voltage to the appliance, which on a switching power supply increases current further, lather rinse repeat.
The part with the pins is scrap, since the arc damage will cause its contact with other plugs to be unreliable, which in turn could cause even more arcing. It won't hurt your Mac, but it might start a fire which would definitely damage your Mac. I would not attempt to dress the pins with a file, as they are plated, and you are exposing the underlying brass, which would then oxidize, worsening contact further. The rest of the charger is fine, you can get "just the little clip-on plug device" by buying any genuine Apple iPad charger, which use the same clip-on. I get mine on eBay for $12.
Or, look at the socket on the power block - that is IEC C8. It can mate with any common "IEC C7" appliance cord. When I had a Macbook PS that had a cord instead of a pin-frob, I left the cord behind and just grabbed a local C7 cord.
The lesson to learn here is don't use AC power sockets that are loose. If the contacts don't grip the plug securely, it is defective and don't use it.
This sort of thing is why UL and the other national testing labs (CSA, TUV, BSI etc.) have very high standards for manufacture of sockets and plugs. Both in terms of contact design and quality, and also the plastics used must resist and not accelerate fire, and not emit toxic smoke if charred by external fire. Such standards do not always exist in the third world, or are not enforced.
The pin location of this arcing is highly unusual. That is not the normal contact point for a UL-listed or CSA-listed NEMA 5-15 socket, which is on the 2 faces of each pin. I suspect you were dealing with one of the infamous Chinese "everything sockets", which make contact with every kind of plug, but very poor contact with any of them. This could not handle 0.5 amps - certainly 16A is out of the question. As such, UL and the major NRTLs will never approve "everything sockets". They cannot be made safely.
"Dual sockets" such as British shaver sockets are possible.