Apparatus for printing postal address code markings

The apparatus includes a conveyor track for passing window envelopes through a read station and a print station. At the read station, an optical sensor (40) reads printed marks signifying the postal code through the window, the information being passed to a computer. At the print station (75) fluorescent or other marks are placed on the envelope, as instructed by the computer. The passage of a mail piece is detected by photocells (24, 26) which switch on the reading function. As disclosed, the print station is composed of two dot matrix print heads (75) which are mutually staggered transversely and longitudinally of the document feed path, whereby full height bars of a bar code may be printed as respectively upper and lower portions by the print loads.