The 9004 is a UHF diode in the acorn format to reduce lead inductance. Acorn valves appeared in the late 1930s as more use was made of the VHF part of the spectrum and smaller inter-electrode capacitances were essential.
The RCA logo. The envelope is 14 mm in diameter and the diode anode is around 5 mm wide.
The white lettering reads Made in USA.
A closer look at the diode electrodes. The active part of the diode is tiny.
The five base pins. When pushed into the base clips the glass was prone to crack and ruin the valve. Later zero insertion force sockets were developed that clamped the pins after insertion.
The thin glass tube envelope is 14 mm in diameter and, excluding the base pins, is 34 mm tall.
References: Data-sheet. Type 9004 was first introduced in 1943. See also1943 adverts.
