I have nothing for you, except a compliment: Nice knife! I've always dug the pure badassery of the US Knuckle Knife. Never seen a REAL one before. Very cool!

 

that is a super find. in nice shape also.

 

Cool knife.

Mine is a craptacular fake.

 

Nice find, congrats!
Here is a photo that shows my US version.....yes, it's the real deal.
I had the Au Lion with scabbard at one point as well.

 

That French scabbard looks perfect for your awesome Au Lion, an original LF&C scabbard is going to be expensive, I've been looking, but would be cool, too.
I have two LF&C without original scabbards.
I think the servicemen who wore them didn't much appreciate the original scabbards.
My two both came with theater made replacements.

One of my LF&C 1918's with it's theater scabbard.

 

I haven't been on here in awhile, this was a cool thread to come across. I paid top dollar back in 2016 for this one, sans sheath but very cool:

I'm very much an amateur WWI history enthusiast, so @THREEDFLYER 's post is making me somewhat jealous.