The 1D8GT is a diode triode pentode. The three sections in a non superhet receiver would provide demodulation af voltage amplifier and output valve in the single envelope.
The RCA logo on the base. The pentode is on the right and on the left the triode is mounted above the diode anode.
The triode face view with the diode below. In the centre can be seen the single filament strand. This is terminated at the top of the triode and a strut connects to a single filament strand descending through the pentode.
The pentode is on the left. The glass is frosted and seeing into the envelope is difficult. Also on the outside of the glass is the American VT-148 code.
The wide glass tube envelope is 29 mm in diameter, and excluding the IO base pins is 68 mm tall.
Reference: Data-sheet, & 1043. Type 1D8GT was first introduced in 1941.