The 6ER5 is a variable μ frame grid tetrode designed for VHF television tuners. The screen grid is there as in the original tetrodes to reduce anode to control grid capacitance. With the screen grid acting as an internal shield the 6ER5 is also known as a triode and is referred to as such on the data sheet.
The Amperex name. The indentation on the anode is there so that the working face is very close to the grid whilst the bulk of the radiating surface is away from the central area.
The side view.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 47 mm tall.
Reference: Datasheet. Type 6ER5 was first introduced in 1960.