The 6BQ7A is an all glass miniature VHF double triode and is internally screened.
A valve like this would be used for RF amplification, or frequency multiplication. In the latter case the valve is operated as a non-linear (Class C) amplifier. This causes harmonics of the signal frequency to appear in the anode current. The anode tuned circuit is then chosen to resonate at one of the harmonics, usually the second or third.
One anode with the screen behind.
This view shows how the screen encloses the right hand triode.
The thin glass tube envelope is 20 mm in diameter and excluding the B9A base pins the valve is 49 mm long.
References: Data-sheet & 1040. Type 6BQ7A was first introduced in 1954.