When you say "the dress can't be done up", you mean the dress is smaller than the mannequin? I doubt the dress is meant to be worn all open like this, with your underwear showing. Are the button holes functional or just for show? If the latter, it supports your suggestion that it was meant to be worn open, but it's unusual especially for the '60s.
Can you please show the underside of the label with the number? Are there any other labels, eg "Made in France", fabric content, washing instructions? Try the waist and side seams.
Your label looks similar to Jonathan's '67 label but (perhaps it's the photo), it doesn't look to be as good quality, with large stitches/weave instead of fine ones. It's also not very well sewn in but that could be because it came out at some point and was resewn.