[Hallicrafters] Tube question for old-timers.

[Waldo Magnuson]

    This morning, while I was waiting for my wife, I was reading "The 
Bible" (otherwise know as "Radios by hallicrafters" by Chuck Dachis) 
and noticed that the tubes used in the Hallicrafters receivers changed 
types around about 1939.  Prior to 1939 the tubes were mostly 6A7, 6C5, 
6D6, 6F5, 6L7  and so forth.  In 1939 there was an "early" and a "late" 
model of the S-19R (according to Dachis).  In the late model a 6SK7 was 
used in place of a 6K7 and a 6SQ7 was used in place of a 6Q7.  Later 
radios used more of the 4-character tubes in place of the 3-character 
tubes.
    My question to the old-timers is:  Were these improvements to the 
same tubes, or just an envelope change, and was there a naming scheme 
used for the changes or upgrades ("S" seems to have been used for some 
of the early transitions).
    Thanks.
73,  Skip Magnuson W7WGM