To what is Mr. Wemmick compared to and why in Chapter 21 in Great Expectations?

Dickens compares Wemmick to a wooden block and a mail box or "post office."  All of Dickens' "mechanical" references in regards to Wemmick demonstrate that this eccentric character is in tight control of his opinion and emotions.  Of course, later Pip discovers that Wemmick is kindhearted, compassionate, and savvy.  However, in this chapter, the only keen observation Pip makes of Wemmick is that

"his mouth was such a post office of a mouth that he had a mechanical appearance of smiling. We had got to the top of Holborn Hill before I knew that it was merely a mechanical appearance, and that he was not smiling at all" (188).

Wemmick has to control himself to avoid any suspicion on Jaggers' part because he knows that his boss has a tendency to take advantage of whatever he knows about his "clients" or employees.