Well, others might have more details, but possibly could be from either Pr. 971, Pr.949 or Pr. 667 subs that were decommissioned in recent times
The DPRK's golden years in acquiring decommissioned surface and submarine vessels, both secondhand and through the design and technology to build them, occurred after the disintegration of the USSR with some of its newly independent states, and during the disastrous economic period of the Russian Federation.
1990s, early 2000s:
MiG-21 Bis, MiG-29s, SU-25s, solid-fuel missile technology, as well as ships including a Krivak and submarines arriving for scrapping, rumored to be conventionally powered Foxtrot and Golf-class submarines, new North Korean anti-aircraft and anti-ship missiles with clear ancestors in the former USSR.
So, are we really sure this nuclear-powered complex is a recent delivery?
Given that Kim Jong-il was presented with a model of a nuclear-powered submarine in the early 1990s, I suspect the DPRK has had this engine for a long time, and not because of the recent alliance signed between Kim Jong Un and Putin. I think logic dictates that a nuclear-powered submarine is designed and built with all the blueprints for the entire propulsion system already available or already in place.
Because the submarine will then have to be designed around this.
So if Kim Jong-un declared a few months ago that a submarine with these characteristics will soon be available, the project cannot have been developed recently.
What Russia can certainly do is provide the expertise needed to implement this technology on submarines and operational use.
