The 6R7G double diode triode was designed for radio receiver use. In the classic superhet broadcast receiver such a valve would provide signal and AGC rectification, followed by the first audio amplification.
The 6SR7 has the same rating but is in a single ended metal envelope.
The National Union name on the base cap. The paper label around the top reads - Quality approved special report test.
The construction above the pinch is typical for the double diode triode used in superhet receivers the triode mounted above the diodes on a common cathode.
Above the central mica the base of the grid is visible. Below is the screen around the diode anodes. The latter are just vertical rods. Below the lower mica the insulated heater is coiled as a double helix that will reduce the mains field around the heater.
The classic envelope is 38 mm in diameter, and excluding the IO base pins, is 100 mm tall.
References: Datasheet, 1040 & 1043. Type 6R7G was first introduced in 1936.