For example, the sixth stroke in 㔔, 㪳, and 㫈. The radical is one single stroke, unlike 口 (a similar radical) which is 3 strokes.
Usage notes
This is a nonstandard stroke found exclusively in three Korean gukja: 㔔, 㪳, 㫈. Its use is to match the hangul letter ㅇ (') to denote the final consonant -ng.