The 3B4 is a miniature RF output pentode designed for Class C operation up to 100 MHz. It is described as directly heated with instant warm-up for use in portable equipment where saving battery life is important.
The stitched anode.
The additional screens can be seen above the top mica and below the lower mica.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 47 mm tall.
Reference: Data-sheet. Type 3B4 was first introduced in 1948.