A full stop (or period in American English) is a punctuation mark that denotes the end of a sentence. The symbol itself derives from Aristophanes of Byzantium who invented a system of punctuation where the height of dot placement determined meaning of a thought or sentence. Until quite recently full stops were commonly used after initials or titles, but punctuation fashions change, and A. A. Milne is now AA Milne, and Mr. and Mrs. are plain old Mr and Mrs (or Ms, more likely). As for a full stop after a single word, I have no explanation, but there is enough of it around on buildings of the late 1800s, especially banks, hotels and civic buildings, that I can only surmise that it was the style of the times.