Definitely a mispell.
The real name is Costo. This is basically a Robust which was rebranded as a failed marketing exercise. The Costos were produced from 1924 to 1940. They are identical to the Robusts and also made by the "Manufacture d'armes de St Etienne". The grade # is lower by one. I.e. a #22 Robust is identical to a #21 Costo.
They are rare but show up once in a while. They often came courtesy of a GI, ususally through Germany.
"Costo" is a brand name based on "costaud" meaning tough, while Robust is a name based on "robuste" meaning... you know what.
Hope this helps.
WC
