The 12AY7 is a double triode designed for low level audio amplifier stages the equivalent 6072A is also marked on the glass. The latter is described as being for industrial and military applications that are critical as to microphonics and where reliability is paramount, ie a special quality valve.
This exhibit from TAD is also marked as highgrade and we also have an 12AY7 marked as selected, but no definition of the selection criteria is presented.
The stage gain of 44 is around half of that obtainable with the 12AX7 type of double triode. This lower gain is deliberate so as to offer an alternative to the 12AX7.
The anode with the working area pressed deep into the metal so as to be close to the grid.
The central electrostatic screen.
The thin glass tube envelope is 21 mm in diameter, and excluding the B9A base pins is 48 mm tall.
Reference: Data-sheet. Type 12AY7 was first introduced in 1949.