The type Z66 was M-OV's late-1940s in-house attempt to make a small, modern high-slope pentode for wide-bandwidth IF systems in TV and radar, where bandwidths of several MHz were required.
A 'pinch' construction was used in order to ensure that the valve would be cheap and easy to make on existing production line machinery. The result was that the Z66 was very much a 'second best when it came to performance, particularly in terms of gain-bandwidth product and high-frequency noise figure.
The view from the pinch end. Apart from the insulated heater passing eight times through the cathode and the copper control grid supports there is little that one can see.
The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 80 mm tall.
Reference: 1040. Type Z66 was first introduced in 1948. See also1948 adverts.