That certainly looks man modified.. I suspect it would be quite unique and not listed in any reference book.
All I can think of is grooves to make hafting something more secure...

Neat

That's a deer bone and it's a natural joint. I could show you on a fresh killed deer. Every so often someone digs one and has the same question. Very unique looking, but completely natural.

After I found it I definitely debated with myself whether it was natural anatomy or not. I work in healthcare & feel I know human anatomy very well(not deer for sure). It looks carved in person & by the shape I know it is a long bone. I don't know of any long bones that are that fat but short. Could be wrong though.....

It's definetly natural! Not sure what species.

Keep investigating B. . . . .
I also have found a number of deer bones with a flat, or "bumpy end. Google; DEER SKELETON DEER BONES. . .top of the list of references was "IMAGES FOR DEER BONES.
About cursur 10% down, left side in blue backround...
3 very good end on pics, called Deer leg bone ( exact name not given in this view but from other pics in same category, probably Metatarsal.)

I didn't think any bones looked similar to your find, hard to see all pics good..could be something like toe / foot bone that is only shown inplace without good end views...

As a matter of passing interest....On that big Metatarsal flat end bone...the OTHER end is radically different....just like a proper door hinge, very intricate. In her
youth, Badger was saving every one of these we found to make some sort of necklace out of them..

Since Badger has so many other things going, making ornaments is not on the list...so she just has a shopping bag of bones ! >

Well I am starting to side with natural as well. Just such a weird joint.... I obviously don't know crap about animal bones. I am disappointed. Not going to throw it out, but more skeptical. Really appreciate the feedback. Thanks guys!