The DB4-1 is a small oscilloscope tube with a short persistence blue phosphor and was designed for oscillography (photographs of the trace).
The tube was made with different colours and each given a separate designation: DG4-1 = green medium persistence, DP4-1 = blue and greenish-yellow for combined short and long persistence.
The gun (left) and electron lens (focus) all held in correct alignment by the ceramic rods. The Aquadag (colloidal graphite) coating obscures the deflection electrodes in the electrostatically deflected CRT.
The 42 mm diameter screen shows no blemishes and the phosphor is very even. This is clearly a tube for professional applications.
The B9G base that was introduced by Philips in 1938.
The end window envelope is 33 mm in diameter, and excluding the B9G base pins is mm tall.
Reference: Datasheet. Type DB4-1 was first introduced in 1953.