The riddle of the North-Korean Nodong missile was difficult to decode up to now. Pakistan and Iran use the Nodong to build there similar Ghauri-I and Shahab-3 missiles. In North Korea the Nodong vehicle was used for the first space-rocket Paektusan (Taepo-Dong 1). In the meantime upon base of an extended Nodong have builded North-Korea, Pakistan and Iran their next variants of missiles.
The North-Korean Nodong missile is not related to the Soviet A-4 (R-2) and R-5M (or the Chinese DF-2) rockets. The use of an engine from a Soviet A-4 or R-5M rocket was not probable, because the used fuel can not be Ethyl-Alcohol/LOX. The flame of the burn-up is apparently similarly a Scud missile. The Scud-B uses TM-185 (Hydrocarbons) / AK-27I (73%HNO3 +27%N2O4) as propellant.
The key to the problem was up to now the kind of the engine used for the Nodong. Recently were found further remarkable photos from the Iranian Shahab/Safir-engine. The engine is clearly similar to the Scud-B engine. It is however ~150% larger formed.
With a meanwhile known diameter the rocket of 1.25 m can be valued a diameter for the nozzle of 62 cm. It is very improbable however, that North-Korea independently has developed a Scud similar stronger new engine.

Similar is a single engine from the clustered Chinese engine YF-2. The engine YF-2 is used for the DF-3 missile. The propellant is however UDMH/AK-27S !
A single Nodong engine has a thrust of approximately 284 kN (s.l.). That can calculated become with the kind of propellant, the quantity of ~11.151 kg and the burning time of 95 sec (Taepo-Dong 1 launch). The calculated thrust approximately agrees with the thrust of a single engine of the DF-3 missile.
Pakistani Ghauri missile have a quantity of 12.912 tonnes propellant. It is calculated a burning time of 110 sec. Presumably then a Ghauri-II missile is meant. The Iranian Shahab-3M (syn. Ghadr-I) has an equal dimension. The new Iranian Ghadr-II missile is once more longer. It is used as first stage for the Safir IRILV. It is based on a Nodong++ missile, which was used probably as the first stage of the unknown North-Korean Unha-1 SLV.
An other interesting reference for the origin of the Nodong  is a Russian drawing for the manufacture of an engine, whose measurements and outlines equal the Nodong engine. It is probable, that this engine belongs to an unknown Soviet  missile R-18 "Scud-D" , which exactly corresponds to the Nodong missile. Possibly the engine was constructed for the use of UDMH as fuel. These missile was not taken over however in the rocket arsenal of the Soviets, probably however exports to North Korea and Pakistan as well as China.

Meanwhile at first by Novosti Kosmonavtiki published sensational photos of the Chinese first stage engine YF-2 from the CZ-1 SLV. It is now confirmed that four single engines are combined with its own turbopumps. The shape of the chambers and the nozzles are more similar to the smaller Scud-B engine than to the Nodong/Scud-D engine.

The Nodong engine has greater similarity to the Soviet four-chamber engine Isayev S2.1000 (Burya booster). On the other hand, the turbopump is much more similar to the Scud-B or the YF-2 engine.
There is some evidence that the Nodong engine is not only identical to the Scud-D engine but also similar to the second stage engine YF-3 from the Chinese CZ-1/DF-4 rocket.

*) F= (M * Isp) / t = (11,151 * 2417) / 95 = 283,8 kN

  Propulsion of the North Korean Nodong missile family

  North Korean Nodong missile (Soviet R-18 "Scud-D")

This rare photo provided by the “Thai Irrawaddy News Magazine“ on June 24, 2009, shows a Nodong missile at a factory allegedly in Pyongyang which was photographed during a visit by a Myanmar government delegation in 2008.

Recently confiscated North-Korean fuel-container for Iran

  Pakistani Ghauri-I and Ghauri-II missile

  Iranian Shahab-3 missile

  Shahab-3M (syn. Ghadr-I) missile