I read the signature below the king's head as Ant. Luyckx. Is he related to Luc Luyckx?
The same thought came up in my mind. It's until chrisild noticed that Luc Luycx' name was written without a K that I noticed it.
Luyckx (with K) is a common name in Flanders, so I wrote Luc Luycx' name wrong for years.
About the 250 francs coin: Antoon Luyckx ( a known sculptor) designed only the obverse. The royal monogram on the reverse was designed by Marc Severin, while the crown above the B-monogram was designed by the heraldry-expert André Toussaint. (ref. "De Belgische Frank", encyclopedia of the Belgian circulation issues)
Even of these coins there are 2 versions, apart from the Dutch and French legends: the regular issues have a reeded edge, while the proof-like issues have 25 stars on the edge.
It took a while to sink in, but I think that explains the white spot on the second coin: it's the CuNi coming through.
Only the first coin (500 francs) is plated. The second coin was struck in 0.835 silver (and 0.165 copper)
Bart