https://hkmdb.com/db/movies/view.mhtml?id=6379&display_set=eng
After years of going over this again and again, looking for any evidence I could to test my theories on this confusing situation, last year I came to a conclusion.
I am convinced that there is a Korean cut of the original HK film from 1982 that was never released and then a re-edit of the original HK print, with new Korean scenes filmed in 1985 featuring a completely different cast (except one) from the "original Korean cut" was created and released in Korean cinemas in 1986. I believe the KMDB has not realised this and has merged detail of both films into one.

As most people are probably aware by now, these HK (and or Taiwan) co-productions usually had a [Korean cut] filmed alongside (or in some cases right after) the original film, with extra scenes for the Korean cast and often featured actors (from Korea or even HK/Taiwan) who don't appear in the HK version. I am convinced that NITDD's was made in much the same way, but for whatever reason, never saw the light of day.
That is, it was never released in cinemas and it has not been released on VHS or DVD. But it does exist and at least one print of the original is safe in the Korean Film Archives. The KMDB has screenshots taken from this print showing the credits from this "original Korean cut". It credits different Koreans than can be seen (and are credited) in the 1985 version, they're all regular stuntmen from Nam Ki-nam's other films (including other co-productions). The only exception is Park Yang-geun who appears in both credits and I can confirm appears in the 85 version.
But it gets more interesting when it comes to one particular credit, the top billed actor on the 82 version, Hwang Jeong-ri. If you've watched the 85 version, you'll know that HJR does not appear at all, which is very strange as he was Korea's biggest action star at that time, why would he be credited on the 82 version but have his scene cut from the 85 version? Obviously it's because that scene features Sanada Hiroyuki, who's entire performance is cut from the 85 version (except when you can't see his face).
It seems that Sanada Hiroyuki was the catalyst for the 82 version's shelving and the re-editing done in 85, but why? Some kind of rights issue? Or perhaps (my top theory) the nationalism and censorship around that time made it a problem to release a film that had a Japanese actor as the lead? It seems the most likely to me, but whatever reason was, Sanada's scenes where cut and therefore HJR was gone too.
This also makes sense of the several years gap between NITDD 1982 release and the release of the 85 re-edit, that I have cinema dates for during May 1986. Sometimes there are seversl months between Hk/Taiwan and Korean cuts of films, but several years points towards a deliberate shelving of the film.

Another interesting image, is that of the actual film reel, as it shows the title 용의 인자, whereas the 85 version's title is 흑룡통첩장, which is shown on the film print in Hanja 黑龍通牒狀. This is also shown on the promo material for the 85 version which shows 흑룡통첩장 as the title, sometimes showing 용의 인자 as an "aka" or "formally". I believe 용의 인자 was the original Korean title, with the name 흑룡통첩장 being created for the 85 version.
There is another piece of evidence (though I'm a bit more sceptical on), the 85 version runs at 1h32mins, but the KOFA has a print in the vaults that runs at 1h24mins. I believe this is the run-time for the 82 version and that they've listed it incorrectly as 흑룡통첩장(일명 용의닌자), as they have merged the two versions as one page. But it is possible it's the 85 version with some cuts, though I think the former is more likely.
I had hoped to try and contact director Nam Ki-nam to ask him directly, but sadly he passed away before I had a chance. It doesn't look like the KOFA is planning to upload NITDD anytime soon, so the only way to be sure would be to go to the archives in person, which the probability of is about zero

I'm convinced and I think the evidence I've provided should be enough to settle the matter, though I'm open to hearing any other theories and thoughts on the subject.
I will eventually create pages for the different version, but for now I'm note the credits with [Korean cut] for the 82 original and [1985 Korean re-edit] for the 85 version.

Cheers