I confirm that the Nieuport of the Brussels Air museum is well a Nieuport 23.
Here is a photo of the assembled aircraft I've taken on 18th of november:
Please note that the pic is copyrighted (myself).
A lot of other pictures will be put on the site of our friend Edmond Thieffry soon; he will keep you posted of course.
Greetings from our little corner of Europe,
Daniel.