WW1-WW-2 or older large sword-bayonet

Description

! have a very old sword-bayonet up for auction.  I have no idea what period it is from, or what war it was used in.  All i can do is try and describe it and hopefully  a collector or military buff will know what it is.  I have been told what it is by a handfull of people     but unfortunately they all  have a different answer!!!
The overall length is 27 inches the blade is 22 inches and has a fuller  running on both sides of the blade, the whole thing has a very heavy patina on it  which leads me to think it is quite old!! The grips look like they may be made from leather ..  I can find no writing just a proofmark on the tang which i tried to photograph as best as i could    I cant tell exactly what  it is, but to me it looks like an old suit of armor helmet............    I would appreciate any info   that anyone can give me.. I will start the bid at .00 if its not worth that i will just hang it on the wall! no  reserve  and .00 to ship it ,lower 48 only..
Thank you