Prose
S. B. U. 21
In plain words
[Not a verse or commentary line -- this is a page marker from the printed book itself: 'S. B. U. 21', most likely a running-head abbreviation with the page number, that was captured along with the surrounding text.]
Close to the Sanskrit
[This record contains no Sanskrit sentence. It reproduces a print signature / page-header mark from the source 1910 edition: the abbreviation 'S. B. U.' followed by the page number '21'.]