Ah yes! My mentality is still back in the days of steel filing cabinets and index card cross indexes to keep everything straight. It is absolutely true that you don't need to make a formal number if you are using computers - the computer program for a data base "automagically" assigns an index number, but you can use google-like search to retrieve any individual document named (for example -"vendor letter offering different forms of electronic payment") and the computer will only allow the most recent version to be retrieved for most users (some researchers/users have special privileges authorizing entry into archives for past versions.) In such case, your footer would only need to reflect a date, since the document name would be in the subject line. Maybe not even a date if your software were robust enough to generate only the most recent version each time a letter is sent to a vendor.
Added in edit:
Some users might want to disguise the fact the document is a form and let the recipient believe it is a unique letter to him.