it's a ww1 french trench knife, they made these from the steel posts used to support the barbed wire & from large nails. i have one too.
also called 'nail' knives. thery come in other variations. all hand made & slightly different. the variant 2, like yours and mine has the central rod opened at the guard so it can be slipped onto a belt easily.
mine is below with a commercially made german trench knife and a cast iron mace head fitted to an entrenching tool handle to make a trench mace.
updated: added another that more closely resembles your viariant. see also another thread here:
http://www.vikingsword.com/vb/showthread.php?t=14232