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 selected, but no definition of the selection criteria.
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 twin box anodes.
The side flanges that attach to the supports are in the centre of the working faces to give the greatest heat sinking benefit.
The thin glass tube envelope is 22 mm in diameter, and excluding the B9A base pins is 48 mm tall.
Reference: Datasheet. Type 12AY7 was first introduced in 1949.