O&S - found at Studland beach

Device found on Studland Beach (Devon, UK) which caused a callout of the gentlemen who deal with such things.
To me, it looks like a submarine launched marker flare. The first line of the French lettering is quite clear - Don't Touch ! I can't see from this image if it has ignited or not.
The old wooden ones had a hole in the tip if they had gone off - there are a couple in the workshop museum.
Not my image, sent to me by someone who knows about the workshop.