One number means square, two means rectangular
A google or two found this:
SmartView FS M300-LT FlatScreen Wall Mount Includes:
37" to 60" LCD and Plasma TVs
Certifications & Standards:
From that, I would suspect that they think they're being clever by saying "Vesa 400" instead of "Vesa 400x400". So, if you see one number, you assume its square, and if you see two numbers its probably rectangular.
So, based on that, your carton means the following: