This is the French Percutante No. 1 defensive hand grenade, referred to in the trenches, as you might imagine, as the ‘pear’ grenade.
The French Army began the Great War, like so many of the other combatant nations, unprepared in the hand grenade department; they were soon using updated 19th Century bombs and petard grenades as well as imported Italian grenades (Italy, at the time, yet to decide on which side it would enter the war), but it would be 1915 before newer designs of French grenade began reaching the Poilus in the trenches.
The Percutante (P1) grenade, designed and manufactured in France by one Louis Léon Billant, who, later in the war, would also come up with the reliable and waterproof ‘Billant’ grenade fuse system, was introduced in May 1915. The body of the grenade, made of pig (crude) iron which featured internal grooves designed to fragment on detonation (see inset photo), was filled with high explosive (cheddite, I believe). This was a percussion grenade (‘percutante’ literally meaning percussive) that needed to land on its base plug (see diagram Fig. 2) in order to explode.
It was delivered to the Poilus with fuse and attached handle (above) already in position, and thus its safety features were of paramount importance. The handle – and this was one of the first grenades to feature a safety handle – was held firm against the body of the grenade by resistance from the spiral spring within the safety cams (see diagram Fig. 2 again), which at the same time safely secured the firing pin within the grenade, and was fastened with a piece of string (not wire, again for safety reasons), to which a small metal seal, missing on my example but visible in the two inset black & white photos above, was attached.
Breaking the metal seal would free the handle which, on throwing, would move through 180°, as above, releasing the firing pin as inertia takes over,…
…and provided it landed on its lead base cap, seen here, the firing pin would at that point strike down on the detonator and the grenade would explode. Provided it landed on its base, that is. And as an extra incentive for it to do so,…
…you might notice a small, but intentional, hole in the handle just below the string, and the inset reveals all. Inside the handle, a cloth (linen?) streamer was attached to this hole, with a small circular plate at the end, as you can see, which would automatically extend when the grenade was thrown, again to encourage it to land the correct way up.
In reality, none of these devices made this grenade particularly efficient, and its small explosive charge, not much more than a couple of ounces (some sources say just over a single ounce), couldn’t have helped. Apparently there were numerous accidents, mainly caused by the inertia-driven igniter which proved sensitive to shock, not what you need in the trenches, and equally, as with any percussion grenade, the mud of trench warfare was hardly conducive to their detonation on impact. So, maybe file under ‘must do better’. Which Monsieur Billant did, his eponymous fuses proving highly effective later in the war.