This article provides examples of NATIONAL UNION-manufactured tubes.
NATIONAL UNION was another quality USA tube manufacturer with most production dating from 1930s to 1950s. Their production numbers were significantly smaller than the big name brands (RCA, Sylvania, GE).
The NATIONAL UNION (EIA) code number is “247″. Here is an example:
(Left = Sentinel brand 6SN7GT; Right = Zenith brand 6SN7GT). Notice the 247 EIA code for National Union on the base of each tube. Both tubes were manufactured in 1951.