From museum Naturalis in Leiden in the Netherlands:
The skull is from the last Ice Age; it is between 100,000 and 10,000 years old.
Pleistocene muskox skulls are very rare.
This one belongs to the two most beautiful ones in Europe, and can now be seen by the public in Naturalis.
See also here.
Daily Leidsch Dagblad of 14 June 2007: only 100 fossil muskox bones in all Dutch collections.