1618 could be the railway inventory number and all whistles of that type would have 1618 on them so that if you needed to order one, you knew which part number to put on the requisition form. It is unlikely that such a minor piece of equipment would be individually numbered, i.e. each one having its own number so that J.Smith was issued with (and probably signed for) number 1618. Or it might just have been the Acme whistle company’s stock number for that particular design; it might just be because whistles supplied to the GCR had the company’s initials on it and that meant a different stock number to ones supplied to other railway companies with their initials stamped on. (Presumably it is the pre-1923 GCR, not the modern one.)