Hi Steady, first, the Luger was made in 1940 by Mauser. The "42" is the ordnance code for Mauser. Your pistol, serial number 6011d was completed in about May 1940. There were about 124,500 Military Mauser Lugers produced in 1940.
The 8,81 is the bore guage measured in millimeters and is the size plug the just fits into the barrel.
Anything else?
Hope this helps!