The PY31 is a half wave HT rectifier for use on AC or DC mains. The 300 mA heater indicates that television receivers were the main application.
The anode is blackened for good heat radiation and the extra width add to the total surface area available for heat loss. The cathode tube can be seen at the bottom.
a closer view of the lower components.
The classic envelope is 40 mm in diameter, and excluding the IO base pins is 88 mm tall.
References: Data-sheet & 1040. Type PY31 was first introduced in 1954. See also1954 adverts.