Up for your consideration is one RCA (Silvertone stamp) & box 6V6 GT USA vacuum tubes black plate. 

The tubes is smoked glass Black plates with clear tops.

Number       6V6 GT (GT = Glass tube)

Brand           RCA
Stamped      Silvertone
Country       USA
Date             1956  
 
Plate            Black
mA               30
Getter          D 
GM               3130 
Tested          62.6
 Rated          88% 

The tube is made by RCA it is stamped  Silvertone.

These are black plates smoked glass with clear tops.

    Date code 274 417 56-52
  • On the TV7 tester 25=65% the value of a new tube.
  •  Each unit of measurement on the Tv 7 for this tube =1.413 
  •  The Tubes tested at 88 % (1.413 x 62.6 = 88.4.% ) 
  •  Pulled from a tube organ.
  •  Made in USA