The EF55 pentode is physically the big brother of the EF50 with which it shares common base connections. The EF55 is a single ended RF pentode with very high mutual conductance and a sharp cut-off. It was designed for video frequency use, probably in RADAR service, and its service code was CV173.
EF55 has the Pentone trade mark and is earlier in manufacture than this exhibit. This is probably from the 1950s.
The current rating places it as a power valve but it is designated for amplifier service with a medium anode impedance.
The black painted aluminium screening can has the CV code on the reverse side. See alsoCV173.
The B9G base with identification codes.
The outer aluminium shield is 33 mm in diameter. The overall length excluding the B9G base pins, is 79 mm.
References: Datasheet & 1040 Type EF55 was first introduced in 1946.