2022, March 16 -- There was probably a failed launch of a big rocket that day. Debris has fallen near Pyongyang. The failure was associated with a huge detonation on midair.
It is possible that this failed launch was the first attempt to test a single-stage HS-17+TR with 6 engines (3x2 combustors), that use as the first stage of the Unha-X.

2019, Dec. 13 -- "Another cruicial test was successful conducted at the Sohae SLG from 22.41 to 22.48 on December 13. The research successes being registered by us in defence science one after another recently will be applied to further bolstering up the reliable strategic nuclear deterrent of the DPRK.”
 A fther hidden static test(s) on December 13 between 21:41 to 21:48 on the Sohae SLG (?).
Comment: The presumed seven minutes burning time is nonsense. This is only a period in which one or more short static tests have been carried out.

2019, Dec. 07 -- "A very important test took place at the Sohae SLG on the afternoon of December 7. The Academy of the National Defence Scienc of the DPRK made a report on the successful test of great significance to the Central Commitee of the Worker's Party."
 A hidden static test of the Unha-X first stage on the Sohae SLG (?)
 Comment: .The engine test stand is untouched. The various small materials (not a container, only a long shadow) are an indication of construction works in the adjacent building during the past days. The old scrap in front of a bunker has not been removed yet. Further presented "evidence" for use the test stand are ridiculous.
NK does not have to test a new engine. They have tested everything they need, even an engine with two combustors (HS-15). Now they only have to test an interaction of several engines (3x2 = 6?) for a SLV. So I suspected a static test on the launch pad. Why are there no current satellite photos of the Sohae SLG.
  

2017 - 2018 -- Latest end of 2017 an analyst should be clear that the engine "Pektusan" primarily is qualified for a future Unha-X, as well as what configuration the future SLV will have. Equipped with the engine "Pektusan" the HS-12, HS-14 and HS-15 have served in addition to the demonstration of supposedly military strength also for the qualification of the engine. In the meantime they master the engine perfectly. It can be equipped with one or two combustors (Pektusan-A1 / -A2) and was even presented a gimbal type engine (Pektusan-B). It is very likely that the HS-15 with the gimbal type engine (Pektusan-B) is identical to a second stage of the Unha-X. For a first stage, three Pektusan-A2 engines may be used, where, like the RD-250, the turbopump is mounted on a platform. For control then four additional smaller engines are required. For a third stage, the engines of the second stage of the Unha-3 are expected to be used.

2014 - 2016 -- Commercial satellite imagery indicates that North Korea preparing the Sohae Satellite Launching Station (“Tongchang-ri”) for launching of a new larger space launch vehicle. The Sohae gantry tower has been undergoing significant modifications. So will be increased the gantry about three sections of 46 m to 60 m (without crane) !
The rocket by a source is described "as a booster for a super ICBM or a heavy-lift space launcher. It is completely new from what they have done so far."
It could be a totally changed space launcher with 3 m diameter and new engines. But North Korea has not the know-how to develop new large engines. That can only mean that already exist the technical requirements for such a propulsion.

Uniformed NADA

Note: What was previously just a guess, seems to be confirmed: North Korea has at least blueprints of an old Soviet ICBM engine. The new North Korean engine "Pektusan" is clearly comparable to the RD-250 engine of the KB Glushko. Two combustors use a common turbopump. Typical is the low arrangement of the TP between the nozzles.
It is pointless to suggest that there is another engine. It is completely clear that there is a pair with a turbopump in the middle, mounted on a platform. Such engines has constructed only Glushko. North Korea have not the know-how to develop an engine of this magnitude itself.
The display (left) shows only one engine for the test. The lowered TP on the platform makes sense only when a second engine is added in future.

"Pektusan 80 tf liquid rocket" is written in red
Translated: Ju Shobo & Han Ho Seok

2017,  December (SEOUL) -- North Korea will launching its new large SLV in September 2018, said a South Korean source. The new rocket would be larger as the Unha-3. A "defector" said the new missile would be considered the "Unha-4". Development of the rocket body is all but complete, but it is expected to take about months to prepare for a launch.
Note:  Probably we already saw the two upper stages of the new SLV in the test. (HS-15)

2017, March 19 -- North Korea has successfully conducted the ground test of its new rocket engine in fully modus (still without the second combustor), added with four steering engines. According to the North Korean news agency KCNA, the test measured the thrust power in the combustion chamber, the structural safety and reliability of the engine, and the movement of the turbine.

2016, Sept. 20 -- North Korea has successfully conducted the ground test of a new rocket engine, the country's state news agency reported.
North Korean leader Kim Jong-Un visited the Sohae Space Center to guide the test of "a new type high-power engine of a carrier rocket for the geo-stationary satellite," the Korean Central News Agency (KCNA) said.
This test was “aimed to make a final confirmation of the feature of combustion chamber, operation accuracy of valves and control systems and structural reliability of the engine during 200 seconds-long working time". North Korea also says the engine has a thrust of 80 tons of force ! However, this refers to twin-combustor arrangement.