Dr Vector's Science Challenge - SOLVED
As Darren postulated, it is a distal caudal (back end of tail) vertebra from a whale. Gray, in this case.
I thought the string and the writing visible on the bottom side (in the original photo; posterior face in real life) would give away the scale, and once you know that... The thing is almost exactly 20 cm (8 inches) in diameter.
The big holes are vascular foramina for the passage of blood vessels. As you can see in the CT image, they go right through the vert.
Sigh. Next time I'll pick something harder. Thanks all for playing. Darren, I have no idea what your prize will be, but it will probably be or involve a lot of beer.