Perhaps a better quality map will solve the problem? As you can see the large scale 1875 map clearly shows a letter box right outside the Ferry Offices. Following the building of Woodside Station and the other buildings alongside the south side of Ferry Approach, the letter box was later moved to the position shown in the postcard below.
For Derek's benefit, I've also included a (poor quality, sorry!) copy of an old engraving of Woodside in about 1850. In it you can just about make out a number of 'turreted' buildings around the top of the Ferry slip. These belonged to the Ferry Company and contained the offices and associated waiting rooms, etc. This style of pseudo-Medieval architecture was extremely popular for a short time in the early 1800s, and similar style buildings appear in old pictures of Birkenhead Ferry, Tranmere Ferry and Rock Ferry, and the same style was used by Jesse Hartley for many of the early buildings associated with Liverpool Docks.