70mm chambers existed in French guns before WWII...I've documented a lot of them but the vast majority of French guns in the period between the wars were 65mm ...2 1/2. 70mm didn't really become common until the 1950's? So I'd expect the gun to be 65mm.
Here is something about Costo from "Passionlachase.fr" with a rough translation below:
Le Costo est né en 1924 lorsque la Manufacture de St Etienne crée une société de vente parallèle (Manumodele) pour pouvoir vendre des armes à un tarif plus élevé en leur donnant une dénomination nouvelle.Les armes sont fabriquées dans les ateliers Manufrance, par les mêmes ouvriers, avec les mêmes matériaux.Sont donc produits des frères jumeaux qui porteront des noms différents.Ainsi le Robust devient le « Costo ».Sa numérotation dans le type est abaissée d’une unité : un Robust n°22 devient un Costo n°21.Par contre le tarif de vente est considérablement augmenté et variable selon les points de vente.
Au point de vue valeur intrinsèque, le Costo est le clone du Robust, il rend donc les mêmes bons services.Son marquage « Costo » est bien plus rare que les Robust et le collectionneur peut y trouver son compte.Sa production cessera en 1940 avec l’extinction de la Manumodele.En salle des ventes le Costo est moins onéreux qu’un Robust, sa décote est même sensible, ce qui peut expliquer son bas prix d’achat à 80e, une affaire donc 8) !Translation:
"Costo was born in 1924 when Manufrance of Saint Etienne created a parallel retail sales company (Manumodele) in order to sell guns at a higher retail price by giving them a new model. The arms were fabricated in the workshops of Manufrance by the same workers with the same materials (as Manufrance Robust). They were therefore twin brothers which carried different names. Thus the Robust became the "Costo". its numbering of the model types was one number less than the Robust...a Robust no. 22 became a Costo no. 21. On the other hand the sales price was considerably augmented and varied according to the place of sale.
"From the point of view of intrinsic value, the Costo is the clone of the Robust, it gives therefore the same good service. It's mark "Costo" is quite a bit rarer than the Robust and the collector can find them with some difficulty. Its production ended in 1940 with the extinction of Manumodele. In the salesroom the Costo was cheaper than the Robust, the offer prices were negotiable, which might explain its lower sales price about 80% (of a Robust). and therefore a bargain."Ie. You have a model number "201" and a serial number 181025. Check out this line: Page 11 - The years of the Robust models....the Robust model equivalent will be "202." which was produced from 1932-1940.
http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=484199
