CV77, or "K tube"

The device measures 50x16mm overall, plus leads. The metal section is 18x14mm plus the screw.

This is described as a cold-cathode thyratron.

The device carries the serial number 4465, which is also written on the box.

The construction appears to be: a metal cylinder with a screw fastening, with a small wire running up within the body of the device. This can just be seen in the photo below. Within the glass section is a metal disc at the top, connected to the blue wire, and a metal ring below that connected to green at the side.