don't have one myself but
picture of the muzzle end, 1886 lebel:
the lebel bayonet appears to seat on the end of the tubular 8 round magazine rather than the protruding rod under it which is quite narrow. (see the pic of the 'legionairre' above). that rod is a stacking 'hook', used in camp to allow the rifles to be tripoded for easy access. the stacking hook was added in the mod. of 1893 and was not found on the earlier 1886 model which required the bayonet be in place to use it's hook for stacking.
i'd guess, even tho i suspect the subject item was based on the lebel bayonet, that it was not actually made to fit one. the grooved top of the grip and the pommel base are wrong. may have been made to fit another rifle entirely, or as you say, no rifle at all. i concurr that if they'd been able to capture a lebel rifle they could have taken the bayonet at the same time (unless they were stealing from an arsenal where the bayonets were kept seperately & they did not have the time or inclination to hunt them up)
must be a bunch of lebel owners out there (bill marsh?) who could comment further.