This is just below the flooring surface in a kitchen and inches from the back door out onto the garden. Camera is looking down - it's an inch and a half long. The 1971 concrete lintel has five of these anchor points, and three don't have wood in anymore. The hole is deeper front to back underneath that it is at the surface.
I think I have to soak some wood that's roughly the right shape then use a clamp or a vice to compress the fat end. Then place it in the hole. Then let it expand to fill the hidden part of the hole, then dry, then shave off the excess somehow.
There could be an alternative using two shaved pieces that can be epoxied into place.
And there could be an alternative with some modern liquidy stuff that sets in place and can take a screw later.
The wooden layer on top of this lintel/anchor would be 1/4" (6mm), and that'd make the subfloor level enough to take laminate/etc for completion of the flooring to the back door.