Agree it's a blazer button with borrowed heraldry.
It’s not the Belgian coat of arms, which would have the motto “L'union fait la force” in French or “Eendracht maakt macht” in Dutch (both meaning “Unity Is Strength”).
The motto seems to be an incorrect rendition of the motto used on the coat of arms of the Kingdom of the Netherlands since 1815. It should read “Je Maintiendrai” (meaning “I will Maintain”) but it seems to be a mis-spelling or variation of that, which confirms it’s not an official button.
The nine provinces in the southern part of the Kingdom, calling themselves “Belgians” seceded from it in the 1830 revolution, leading to an independent Belgium finally being recognised in 1839. Belgium used essentially the same armorial as the Netherlands did prior to the separation of the southern provinces, but with one or other of the “Unity Is Strength” mottoes in reference to the solidarity of the now-independent provinces that united to become modern Belgium.
