I've never seen one myself, but at a guess, there are two options.
First, it might be for a second marketing campaign.
I have the photo-ready ad sheets (the kind you see in the movie section of your local newspaper) for Ghostbusters, which says “coming to save the world again”.
I do recall the movie being in theaters beyond the summer release (it having been such a hit - plus they were attempting to gain attention prior to the Oscars).
I'm just wondering, why do you call it an “advance” poster?
The second option is that posters were produced for a release later than the summer. However, everything we've ever seen suggests that the movie was always intended for a summer release. It's possible posters were produced, just in case the movie couldn't meet the rushed completion date, but you know what they say about complicated answers.
The third option is that the poster is for an overseas release (like the UK - it is in English after all). However, UK posters are more often than not horizontal. I'm assuming, since you didn't mention it, that it's a typical vertical poster.
I'd have to check to be sure, but I'm guessing you have the first option. Really cool poster! Nice find. Where did you find it?