For example, in Verses upon the duke of Buckinghams returne from the Ile of Rees (https://www.english.cam.ac.uk/ceres/ehoc/lessons/lesson1/index.html) the poet spells "art" as "ar't" in the phrase "And ar't return'd again with all thy faults..."
What is that apostrophe eliding?
Addition: I see that the OED shows "art" as the second person singular (archaic) of the verb "to be", with a citation "1535 Bible (Coverdale) Psalms cxxvii. 2 O well is the, happie art thou." So there is a use with the following "thou", so "ar't" can't be a contraction of "are thou".
My thoughts are converging on the apostrophe being a mistake (much like so many people now use apostrophes in plurals - "Pad's available") caused by the writer thinking that "art" is a contraction of "art thou". Shakespeare uses "Art cold?", "art mad?" in King Lear without the apostrophe (ref First Folio), but later writers quoting King Lear insert the apostrophe for some reason (cf Wells, Poetic Imagery, 1924).