Hugh W. Nibley explains significance of "by his own hand" in BOA header.
Hugh Nibley, “As Things Stand at the Moment,” BYU Studies 9, no. 1 (1969): 78
So when we read "The Book of Abraham, written by his own hand upon papyrus," we are to understand, as the Mormons always have, that this book no matter how often "renewed" is still the writing of Abraham and no one else; for he commissioned it or "according to the accepted Egyptian expression" wrote it himself—with his own hand. And when Abraham tells us, "That you may have an understanding of these gods, I have given you the fashioin of them in the figures at the beginning," we do not need to imagine the patriarch himself personally drawing the very sketches we have before us. In fact, the remark may well be the insertion of a later scribe. To the Egyptian or Hebrew mind the sketches could be twenty-seventh hand and still be the authentic originals, as long as Abraham originally ordered them and put his name to them.