Hello Dave, the crown/crown U proof marking was applied by the official proof house in Württemberg, where Mauser was located. It was legally required to have all guns test fired with an over pressure test round in order to check if the gun is safe and reliable. After passing the test, the gun is marked ('proofed') to signify it underwent the test with good results.
Other German states used a crown/N proof marking. Both the crown/N and the crown/crown/U were followed by an eagle/N in 1940. After WW2, West-Germany introduced a variation of the eagle/N as a proof marking, and only just recently a few years ago the eagle/N was followed by a CIP/N marking.