Dismantling the bomb and managing the nuclear materials : summary. (OCLC #29195498)
The AACR2 record for this title includes the note:
500 __ ǂa "September 1993"--P. [4] of cover.
which means that the quoted phrase appears on the cover on the back of the piece. The cover is logically paginated: 1 (front of front cover), 2 (back of front cover), 3 (inside of back cover), 4 (outside of back cover). Since the cover doesn’t actually have those page numbers printed on it, they are bracketed in the note.
In RDA, the LC-PCC policy statement for punctuation in notes says to not use square brackets in notes except when they are used in quoted data, so the RDA record would have this note:
500 __ ǂa "September 1993"--P. 4 of cover.