The PL81 was designed for universal mains sets without a mains transformer and with the heaters on a series chain. The parameters given are for a pair in push pull audio service.
As a line output valve in television service, the peak anode surge voltage is 7,000 Volts. The anode dissipation is 8 Watts.
Mullard also specify the PL81 for use as a series pass element in voltage regulators.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 71 mm tall.
References: Data-sheet & 1040. Type PL81 was first introduced in 1951. See also 1951 adverts.