The 6R7 is a double diode triode. The 6R7 was designed for radio receiver use and in the classic superhet broadcast receiver such a valve would provide signal and AGC rectification, followed by the voltage amplifier triode to feed the output stage.
In the 1940's the 6R7 was re-designed into a single ended version without the top cap, this is the 6SR7. The 'S' indicating a single ended version.
The Philco advert to encourage repeat business.
The Philco name.
The top cap connects to the control grid of the triode. Pin 6 is not used indicating that the high impedance connection to the grid was being kept well away from the AC on the heater to minimise hum pick-up.
Under the plastic base are the lead-out wires from the glass disc that is sealed into the metal envelope.
The thin metal tube envelope is 25 mm in diameter, and excluding the IO base pins, is 62 mm tall.
References: Data-sheet, 1040 & 1043. Type 6R7 was first introduced in 1936.