If your trying to duplicate the numeral impression, use a block of pure lead behind the sheetmetal when you strike a chracter die, for a "factory" looking appearance ....McMaster Carr.com if you have access, has many lettered and numeral die sets, individual chracters and sets, custom made dies are available too, esp if you want to duplicate a stamped multiable sequence number repeatately, you can have a multi character 1 peice die stamp made to your spec, font size, standard and reversible for around $25-$100 depending on your requirements, rather than stamp the characters individual for a professional appearance
Mike