Based a series of pictures from an expired ebay.de auction showing the same gun (25 mm CAM modèle 1940 Hotchkiss) and ship in the background, it looks like it was a French ad-hoc mounting on a beach at Calais in 1940. It reminds me of the mounting of the 95 mm Lahitolle on oak beams (French "batteries de circonstance").

Emmanuel