貝克街221B的B是什麼意思？

以前以為這個b就是某某廣場a座這樣的分號，但是有聽說這個b是表示兩人間，也有說是有兩戶人家的意思。請問是這樣嗎？還有沒有類似的例子嗎？

221B Baker Street is the London address of the fictional detective Sherlock Holmes, created by author Sir Arthur Conan Doyle. In the United Kingdom, postal addresses with a number followed by a letter may indicate a separate address within a larger, often residential building. Baker Street in Holmes" time was a high-class residential district, and Holmes" apartment was probably part of a Georgian terrace.
At the time the Holmes stories were published, addresses in Baker Street did not go as high as 221. Baker Street was later extended, and in 1932 the Abbey National Building Society moved into premises at 219–229 Baker Street.

—— http://en.wikipedia.org/wiki/221B_Baker_Stre簡單說，221B Baker Street就像國內門牌號碼的「長安街100號甲」或者「廣州路122-2號」這種附屬於大型居住區的分號。並且在柯南道爾寫小說的時候，貝克街還沒有221號，直到1932年，貝克街向北延伸，才出現了221號這個地址。