The Tungsram APV4200 is an indirectly heated full wave rectifier.
The fine coiled heater wire passes into ceramic tubes below the lower mica and there are two horizontal rails that the heater connects to. Thus both sections are run in parallel.
Side view.
The dome carries the identification and basic operating details. The description is that it is a barium rectifier.
The classic envelope is 39 mm in diameter and, excluding the B4 base pins, is 91 mm tall.
Reference: 1043. Type APV4200 was first introduced in 1933. See also1933 adverts.