Unproven maybe, but North Korea have certainly been working very hard on MLRS.
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Unique in the world they employ 600mm super heavy MLRS. In North Korean military theory with increasing number of K9 and other advanced guns facing them on the 38th parallel tube artillery can no longer reliably deal out destruction against the south before they are destroyed. Therefore shoot and scoot and the ability to deliver very heavy fire power in very narrow time window is important to them. So they focus a lot on MLRS including super heavy ones.
Once Russia starts buying military gear from North Korea openly it also opens up avenues for sudden breakthroughs by North Korea where they "invent" familiar looking weapons and immediately offer them for export to the Russians.