The 6AU6A differs from the 6AU6 sharp cut-off pentode in that the heater is controlled such that, when used with other similar valves, heater voltage surges are minimised and the warm-up time is defined.
Tektronix oscilloscopes were regarded as some of the best made and they took great care over quality control as can be seen here. A Tektronix oscilloscope would be supplied with a small coil of silver loaded solder to ensure that any modifications/repairs were made with the same standard of joint conductivity.
The General Electric GE logo. The three wire grids can be glimpsed through the hole in the anode.
The thin glass tube envelope is 18 mm in diameter and excluding the B7G base pins the valve is 47 mm tall.
Reference: Datasheet. Type 6AU6A was first introduced in 1958.