rev. = “revised”, meaning that a significant element was added after the listed initial completion date (ex. another paragraph in an essay); this is very rare.

pr. = “proofread”, meaning that minor elements were changed (i.e. added, removed, corrected, or otherwise altered) after the listed initial completion date (ex. a word replaced with a synonym, or a misspelling corrected); this is common.

♦ = “end”/“Q.E.D.”, signifying that an essay is done; this is more formally called a colophon, except it’s not (the real name is “tombstone”), but colophon sounds cooler to me.