Ah, but can we be SURE that the Guides were not written by more than one Butterworth in which case the apostrophe would be AFTER the 's'.
Absolutely. You are, of course, bang on.
As for my part, whether by accident or design, I still couldn't help a nifty chuckle when I seen the apostrophe-less "Butterworths" and "Grammar" at the same time........... (you know what I mean).
I'm perhaps just stabbing in the dark here, but do you get the impression - albeit a teeny-weeny one - that our man Down Under (Derek) is really a pen-name/pseudonym for Mssr. Tulloch?
Derek:
If you are earwigging, and if there is indeed any weight behind my suspicions , then may I be so bold as to ask you for: 1) your autograph, and 2) a free copy of your book?
(I'll even put you in contact with a very learned modeller for in return that you dare be so brave as to suggest to some folk that they might start inserting a comma just prior to writing, say, any name. They tend to get really, REALLY irked if you mention that particular one.)