A postage meter saw on a mail from Timbropresse, Timbres magazine's publisher:
In black, Toshiba machine's points can be recognized. The letter was sent on 28 April 2009 and redated on the morrow.
Mail franked with meter must be deposed at a post office the same day. Either the sender put this mail on the 29 April, and the Toshiba date is a correction from La Poste. Or La Poste mail system failed to deliver it on day+1 and confirm the treatment on the 29 April by this second date?
I think the first hypothesis is the correct one. I believed I saw such an example on one of the blogs I regularly read, but, of course in this sort of occasion, I could not put my eyes on it again.