Not any "known" OS to me, it just seems something made for the device, very basic. If anyone wants some information, I got the device, but didn't use much so far, as I got a PRS 650 for reading books. I intend to use the device mainly for watching videos, and am currently trying to convert my videos (tv recorded programs, TS /mpeg2) to something the reader can read.
Oh, and it's the same device as the one from this other thread, and is commonly sold around 80$ (that's the price I paid).
https://www.mobileread.com/forums/showthread.php?t=97896