The plot thickens!
Yes, it could certainly be darker heartwood I'm seeing. Aleksei spotting a standard wood wedge in there is something I wanted to see there too but was trying not to jump on myself. It could be one if the fit of it was perfect and now the wood has aged so it's not so noticeable.
Looking at it again, I see the dark section is squared off at the flatter sides of the handle, this could be an indication that the heartwood is in the center of an inserted wedge.

Thanks Peter J,
You don't happen to have any photos on the iron wedges or if you can list the finds I can talk to the museum to get some myself?

I did an experimental flush profiled wedging of one of the axes. The wedge was a little less wide than the handle top though so it doesn't go all the way at the sides. Next one will go all the way and when I round off the sides it hides it, just like at the top.

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The wedge goes 2/3 through the eye and the cut for the wedge is also that depth or just slightly more but still inside the eye only.

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This is one possibility of what we're seeing in the Oseberga photos. This turned out to be a very secure and solid wedging as well as attractive looking.

In the absence of a specific origional with an obvious conical section suggesting a handle inserted from the top then wood and steel wedges seem the answer .
steel as well as wood wedges are definatly a belt and braces answer and I think they would have been used by some and not by others. It is my experience that they work better than wood alone .
Absence of steel wedges would not mean they were not used on a specific axe , I am so cheap I often re use my wedges when a handle breaks on one of my tools!!
these are good questions to ask.

as an aside I find wedges fascinating , they have many many uses in antiquity across many crafts and are not so prevalent today as they once were.