This box would have held HE/fragmentation shells for a French 60mm Modele 35 mortar. Nearly 5000 of these were in service with the French Army in 1940 ; the Germans captured many and called them 6cm Granatenwerfer 225(f). The design was used by the US as the 60mm Mortar M1.

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"Stand by to pull me out of the seat if I get hit" - Guy Gibson