Are these drawings electronic (.CAD, .DXF, .IGS etc) or scanned paper?
If electronic then why not just have an Alias system. Give each drawing it's own Alias with your in-house numbering system. When a revision comes in give it a dash-number to the original Alias. Print it with whatever border you like but reference the customer numbering and revision dates.
If it is scanned paper drawings then it is nuts to scan them -- unless -- each new version from the customer is scanned over the preceding one so there is no record of the previous out-dated version. Well, other than backed-up archive versions that are generally not available to the user (draftsman) base.