A witness who is not otherwise qualified to testify as an expert may state his opinion whether a writing is in the handwriting of a supposed writer if the court finds that he has personal knowledge of the handwriting of the supposed writer. Such personal knowlegde may be acquired from:
(a) Having seen the supposed writer write;
(b) Having seen a writing purporting to be in the handwriting of the supposed writer and upon which the supposed writer has acted or been charged;
(c) Having received letters in the due course of mail purporting to be from the supposed writer in response to letters duly addressed and mailed by him to the supposed writer; or
(d) Any other means of obtaining personal knowledge of the handwriting of the supposed writer.