The CV3797 is a caesium photocell with a common anode rod and twin semi-circular photo cathodes. The bottom cathode is k(1) and the top k(2). The commercial equivalent is the V944A and this is quoted in the specification.
The stacked cathodes are seen side on.
The front view of the photocell.
The wide glass tube envelope is 25 mm in diameter and, excluding the IO base pins, is 76 mm tall.