The DH7-91 is a high sensitivity green screen oscilloscope tube. For a tube with a small screen diameter it is quite long, and it is this shallow deflection angle that enhances the deflection sensitivity.
The electrodes are held in glass rods and the gun is held in place in the envelope by the springs that can be seen on the right. The flaring deflection plates on the left are the Y plates. These are always closest to the gun as that is where the sensitivity is the greatest.
The phosphor covers the screen and extends around the end of the envelope. This gives an even coating for the whole screen. The black area is the colloidal graphite coating that forms part of the final anode. Commercially colliodal graphite was known as Aquadag. The screen is 68 mm in diameter.
The end window envelope is 40 mm in diameter, and excluding the B9G base pins is 232 mm long.
Reference: Datasheet. Type DH7-91 was first introduced in 1960.