Explaining British punctuation, Oxford Dictionaries writes: “If an abbreviation consists of the first and last letters of a word, you don’t need to use a full stop at the end,” and its examples are “Mr”, “Ltd”, and “Dr”.  Actually the abbreviation “Ltd” doesn’t consist of the first and last letters of the word, although it uses them.  But maybe fine distinctions of terminology aren’t what the Oxford Dictionaries are about.