There were a few news stories last week about North Korean drones (in South Korea), but I didn't notice the linguistic connection until now.

According to the Washington Post, for example:

South Korean officials suspect the drones were from North Korea because Korean-language letters on their batteries are written in North Korean style, [Defense Ministry spokesman] Kwon [Kihyeon] said.

Chosun Online reports:

坡州に墜落した無人機は、エンジンのバッテリーの裏にハングルで「起用日」「使用中止日」と書かれていることが分かった。「日」という文字は韓国とは異なる北朝鮮式の表記だった。

Roughly: "It was learned that the drone that crashed had "起用日" and "使用中止日" written on the batteries on its engine in Hangul. The character [corresponding to] "日" was written in the North Korean orthography, which differs from the South Korean."

An article from the JoongAng Ilbo further reports that "起用日" is not a word in use in South Korea at all, although they assume it means the day when the product began to be used.

The orthography in question was apparently "기용날자"; you can see a picture here. The South Korean way to write this would, I am given to understand, be "기용날짜". So the spelling is equivalent to nalja (North) or naljja (South), I guess. This seems to be a case of what Wikipedia calls Indication of tensed consonants after word endings that end with ㄹ, although that very heading appears to be a mistranslation: the Japanese version of that section clearly specifies word endings that include ㄹ, and notes that the two examples given are not exhaustive. In any case it appears to be a purely orthographic difference.

(Note: After figuring this out, I was able to search for "nalja" and "naljja" and found this article at the Kyunghyan Shinmun, which seems to back up what I pieced together. Whew.)

Any Koreanists in the audience want to correct and/or elaborate on this? For example, where did this difference come from in the first place — does it reflect regional variation or uneven evolution?