As has been stated it is a Tango-Down-PR-4-Rear-Sling-Mount. It is pricey but it's a great mount. It's bolt-on and doesn't require any weapon disassembly.
That Tango Down QD mount is quiet expensive although It does looks high quality. For a little more you could get a PWS buffer tube with built in QD mounts and also eliminate the castle nut.
Im happy with mine even though it's overkill but im a sucker for well made solid solutions.