6J7MG  = 6J7GT  + metal shield;

6J5MG  = 6J5GT  + metal shield;

check the photos below:

6J5MG (metal shell removed)       6J5MG

pin 1 is connected to the metal shell 

if you connect pin 1 to ground of your amp, the tube will be  free from outer electromagnetic waves or glitch or noise.