The tuning eye using 2 squares is type 6AL7. It is more accurate in FM circuits as it can indicate the zero voltage center tuning in the FM discriminator. One control electrode is grounded, so that square will remain at fixed height, which indicates zero volts. The other electrode is connected to the FM discriminator, and that square will vary in height above and below the height of the other square as the set is tuned. Correct tuning occurs when both squares are equal in height (see image below), indicating the discriminator voltage is zero and the circuit is balanced (tuning eye tubes will work only in split-sound TV circuits - they will not work in intercarrier sets, since the sound IF frequency remains constant as the fine tuning is varied).

The 6AL7 is the eye tube used in DuMont sets. Allen B. DuMont invented the eye tube in the early 1930's, and sold the patent to RCA for $20,000. He used the money to purchase the factory building in Passaic, NJ where early DuMont TV sets and CRT's were manufactured.