Thank you Warguy, I'll definitely look into it. What I was referring were rare examples of open collar US uniforms worn by pilots in the great war, I know it might sound strange but if you look at certain photos from ww1 squadrons you can come across some examples of those tunics, here's a few (mostly from the 94th aero squadron):    

The reason any hard rubber replica military training knife won't fit in an original sheath is the tips of the hard rubber training knives are much wider than the original knife's. 

I found a plastic m3 knife by the way from Soldier of Fortune https://www.sofmilitary.co.uk/plastic-ww2-m3-knife-with-brown-handle-us-m3-plastic-knife.html Does anyone have any experience with them? is this a good option that will fit into the scabbard?

I know the KA-BAR Wouldn't fit, But it was nice to see an option for one of those too. Thanks for if you end up finding a good one too!

Yeah I enjoy looking at other people's "junk" and how they decorate. I've gotten some good ideas that way.    

I believe they were issued as separate items. It was up to the individual soldier to attach or have them them attached if so desired. So, various methods of attachment can be found on period examples. Though wire loops were the norm, pins were were added, again based on the soldier's preference.

Correction, if I found it at a gun show I would also steal it for $35.

A replica KA-BAR won't fit an M8 scabbard even if dimensions are true. If I see a rubber M3 I'll remember this post.

That is a beautiful collection. The bayonet looks amazing mounted. That guy probably paid $35 for a cut down fighting knife. Here is mine, I have yet to find an original but when I do I couldn't bring myself to steel it for $35, unless it was up for auction. 

Ah I see, I'm looking for one that will fit, but thanks for sharing!