The STC 3A/146J is a UHF triode designed for grounded grid use. The UK government procurement specification is as CV53 which is also marked on the envelope. See alsoCV53 that has the spade connectors on the connecting leads.
It was around 1940 that STC produced the first disc seal valves. The 3A/146J is one of the first successful production designs.
The grid is connected to the flange but is not a mesh stretched across it.
The side view across the flange.
The anode dissipation in natural air was 2 Watts. The amplification factor is 100 and to derive the operating bias the cathode resistor was specified as 150 Ohms.
The electrodes in close-up reveal the fine grid wire and its close spacing.
The wide glass tube disc seal envelope is 27 mm in diameter, and excluding flying leads is 65 mm tall. The flange is 48 mm in diameter.
References: Data-sheet & 1040. Type 3A/146J was first introduced in 1947.