National Union was a major player in the 30's through the early 50's. Civilian tubes from the WWII years had the code MR meaning it was not war critical and was authorized for 'Maintenance and Repair' in the civilian world.
A lot of oldtimers I knew used them frequently. National Union made many other electronic components as advertised on their tube boxes. They did not have anything to do with the National brand begun by Varian and now owned by Richardson.
The tube writing progressed from etched (hot stamped) to silver to a white with a definite blue tinge. They often had 2 letter date codes on their tubes, in addition to the MR code.
One era's version of their tubes had a small box on the tube base in which to write the date sold for the tube. These were the blue tinged written tubes.
The time line on the tube boxes we have appears to run from your left to right.
National Union Poster

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Last Update: 2/21/2012
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