National Union manufactured tubes and even rebranded them for others. You can find Zenith branded 6SN7 tubes which were made by National Union. A NU manufactured tube will have a tube code of 247 somewhere on the base of the tube.
Here are two National Union tubes, branded Zenith and Sentinel. Both show a 247 code and both were made in 1951.
http://tubesound.com/wp-content/uploads/2007/12/247.jpg