The M84 is a modern iconic type of grenade, a well known silhouette in itself as a highly successful flashbang / stun grenade as used by the US military.
This model is just a fake, nothing more then a well crafted replica dummy model. For display purpose only, this all metal model looks and feels very realistic. The pins are crimped to prevent accidental removal.
Grenades like this are great for filling out vests, displaying on shelves or for theatrical use as they are of course far safer then actual detonating devices.