Not On Label pseudo-labels have been created to gather distinct series of releases without labels (RSG §4.4.2.).
A solitary release is not a distinct series, so simply Not On Label is fine.
For a distinct series of releases self-released by an artist, please use: Not On Label (ArtistName Self-released)
and replace "ArtistName" with the actual artist name. (Please note specificities in RSG §4.4.3.).
Related guideline: Not On Label
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