The Type 58 is a variable μ RF and IF pentode for use in superhet radio receivers and dates from the late 1930s. Electrically it is identical to the 6D6 and the 6U7G.
The RVC logo with the Type designation in the outer ring.
The top cap and between the screen and the inner coating can be glimpsed the white oxide coated cathode and the three wire grids.
The classic envelope is 38 mm in diameter, and excluding the UX6 base pins is 102 mm tall.
References: Data-sheet, 1040 & 1043. Type 58 was first introduced in 1936. See also1936 adverts.