I ran across this cartoon this morning, [Link]
and could not resist commenting. Any who have dwelt on an installation of the Yankee army know that the YA uses a combination system for numbering its buildings. They map the installation off in squares of a given area and number then sequentially (raster scan) with the last digit zero (xyz0, e.g.,) and then number the buildings 1-9 within the square in the order they are built. If they build more than 9 buildings in a square then the appended letters come out.
Maybe this will help you understand the joys of YA mail service.