Not a expert but, dont think its natural, i do see lines running with the groove but i think it could have been worked on after it broke. maby tried to be reused for something else.
Don't see much evidence of drilling in the hole, but still looks like a banner to me. Here's one that I found where the hole was enlarged by gouging, totally obliterating all boring marks, so I wouldn't be so quick to shoot it down as naturally formed, especially considering the material that it's made from. Looks like you're on a good site, Willjo.
I think I have another broke that exhibits the same properties. I'll see if I can find it and post a picture of it later.
The reason the hole does not look drilled is after it was drilled they came back and polished the hole or straightened it. The polished marks show this.
If you enlarge the photo of the broken banner you can still see some of the drill marks. These things had a shaft stuck through them and it had to move, so long term use would give it a polished look. No one knows for sure what they were used for but JMO I say a weight for a drill system.(just throwing that out there).
Nice find willjo, many have hunted all their lives and never found an axe or celt.
But to find a piece of a banner stone is all the more unlikely. It's not hollow core ground, so,the hole does not show that particular style of hole grinding.
Nice find! Your's is solid core ground.
Your Bannerstone would be classified as a "winged" type or a "butterfly" type Bannerstone.
There were two different methods used for forming the holes, and both were used contemporaneously (at the same time) on winged and butterfly Bannerstones. The hollow core method leaves thread scars in the hole, the solid core method used chert tools to make the hole.
They sometimes left vertical scars through the length of the hole, but no thread scars.
The number of archeological sites that have rendered winged and butterfly Bannerstones along with atlatl hooks (bone or antler components of a spear throwing device) has demonstrated that Bannerstones were used as counterweights on Atlatl shafts. I see nothing whatsoever unusual about the hole on your solid core drilled Bannerstone fragment.