When any document, purporting or proved to be thirty years old, is produced from any custody which the court in the particular case considers proper, the court may presume that the signature and every other part of that document, which purports to be in the handwriting of any particular person, is in that person's handwriting and, in the case of a document executed or attested, that it was duly executed and attested by the persons by whom it purports to be executed and attested.

Explanation." Documents are said to be in proper custody if they are in the place in which, and under the care of the person with whom, they would naturally be, but no custody is improper if it is proved to have had a legitimate origin, or if the circumstances of the particular case are such as to render such an origin probable. This explanation applies also to section 80.