The 6BQ5 is another designation for the Mullard designed output pentode EL84.
The envelope is discoloured - a sign of a hard life.
A μ-tracer plot for this exhibit. This shows a good shape to the curves, in many cases a plot of a used EL84 type of valve will be far from the correct shape.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 70 mm tall.
References: Data-sheet, 3002 & 1040. Type 6BQ5 was first introduced in 1953. See also1953 adverts.
