The H63 Marconi triode is a high μ AF voltage amplifier triode for use in RC circuits. The elliptical lozenge with the identification is typical M-OV.
Our other H63 has a glass tubular envelope and the internal structure is more easily seen.
The Marconi brand label together with the Marconiphone company name. The Marconiphone company was the consumer arm.
The mica sheets at the top and sides hold the triode firmly in the shoulder of the envelope. The top mica shows two sets of grid supports and indeed there are two sets of grids. The inner grid connects to the top cap. The outer grid is connected to the anode below the lower mica.
The classic envelope is 38 mm in diameter and, excluding the IO base pins, is 97 mm tall.
References: Data-sheet, 1040 & 1043. Type H63 was first introduced in 1937.