(iii)the signature and every other part of such books of account or other documents which purport to be in the handwriting of any particular person or which may reasonably be assumed to have been signed by, or to be in the handwriting of, any particular person, are in that person's handwriting, and in the case of a document stamped, executed or attested, that it was duly stamped and executed or attested by the person by whom it purports to have been so executed or attested.]