The US Post system has defined the Zebra code. This is a series of diagonal lines to the right of the barcode that serves solely as a visual indication that a tray contains bar-coded mail. The code must not appear on tray labels for non bar-coded mail.
This standard is applicable since July 1997.
Due to its simplicity, the Zebra code is implemented in BarDIMM as a font with just one diagonal thick bar coded in the slash character ‘/’ (ASCII value 47).
Therefore, you have to call the font and send three consecutive slash characters without any space in between. Here are the PCL codes to send:
< Esc >(10U< Esc >(s0p2.50h29vsb23591T///