I apologise if this has been asked before, as I just skimmed through the pages, but in the British editions of the books are the abbreviations spelled with periods like they are in the Canadian editions (the only copies I have) or without?

NEWTs or N.E.W.T.s
OWLS or O.W.L.s

I've also seen it spelled S.P.E.W. with a period at the end, even though the abbreviation wasn't placed at the end of the sentence.