The 5B/254M miniature beam tetrode was designed to have the same ratings as the classic 807. The difference in size between the 807 and this valve shows the extent to which valve technology matured during WWII and the effect of the introduction of the pressed glass base. This particular base was the earliest to be developed with eight equi-spaced pins and an external metal collar to hide the envelope seal and provide a locating spigot.
The valve is also very similar to the STC 5B/251M and electrically identical to the 5B/255M. The latter has the anode connected at the base and one less connection to the cathode.
The end of the anode with the internal beam plate visible. Also note the radiating fins on the control grid supports.
The valve also carries the CV designation of CV428. The grid wires visible above the anode are in fact both control grid and screen grid as the two are horizontally aligned.
The wide glass tube envelope is 28 mm in diameter and, excluding the B8B base pins, is 86 mm tall.
References: Data-sheet & 1040. Type 5B/254M was first introduced in 1957. See also1957 adverts.