The CV3998 is a high-slope RF beam tetrode primarily intended for use in wideband amplifiers in telephone carrier systems, radar equipment and measuring equipment. The CV specification shows that the Government requirement was for the GPO telephone system. The prototypes were the 5A/170K from STC and the E180F from Mullard.
The very small size of the actual electrodes helps to reduce inter electrode capacitance and thereby raises the maximum usable frequency that the device can operate at.
The blackened anode has been formed into a plate either side of the grids with a strap holding them together. The support of the suppressor grid can be seen and the notch and peen wire fixing system is clearly seen in the image.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 38 mm tall.
References: Data-sheet & 1040. Type CV3998 was first introduced in 1957.