In my opinion, "Residents' Parking Only" says "Parking only for residents" (which would be the correct wording for a sign), while "Residents Parking Only" say "Only Residents are parking here". Does that make sense?
The issue here us not the positioning of the apostrophe but whether an apostrophe is needed at all. It depends on whether the possessive sense is being used.
Confusion often occurs when there is plural or word ending in S and a possible possesive. For example when referring to a football shirt seen at Elland Rd it is a Leeds shirt not a Leeds' shirt as you would say a United kit not a United's kit. Similarly we refer to a station for trains as Train Stations not Trains' Station.
So what would it be if there were only one resident and one parking space. You would say Resident's Parking space of it belongs to him. But if you considered 'resident' to be an adjective you could describe it is a resident parking space.
It is a grey area and my advice is, if in doubt, rephrase it so there is defintely no need for an apostrophe- so in Hopkirk's example the sign could say Resident Parking only or Parking-Residents only
On first "scribblings," I admit to missing a couple. In speech, sometimes they do not sound too bad, but when you go back and correct them, the text usually reads much better.
"To partly split the infinitive is surely crass and would it not be better to split the infinitive partly."
The issue here us not the positioning of the apostrophe but whether an apostrophe is needed at all.
Thank you Factor, that was what I was trying to say.
AYG, like me, you were probably flicking ink apostrophe pellets off the end of a ruler, at the back of the class. I was gobsmacked when I saw my English Lang and Lit O level grades!