Hello,
Bump this thread.
Looks like a "pétard", "artifice" or "signal" for explosion simulation or something similar. Typical shape (card-paper, black cover and white markings, safety fuse). Maybe 30's era.
The narrowing (venturi role) at the base could also match with the stick (propellant) of signal ("fusée de signal") ; some kind of small paper rocket signal (in this case your device could be incomplete).
Could you give dimensions (length and diameter) ?
Can you see other markings after ECP (lot and date) ?
Regards
Herewith a example of "Fusée signal" (marking ECP and maybe 1921) - no details on this grenade (use - filling) :
View attachment 161732
Cutaway of another one :
View attachment 161733