The 6BX6 is equivalent to the Mullard EF80. This exhibit was made in Japan and is branded Denon. Denon make electronic equipment and we can find no record of valve production.
The 6BX6 is an internally screened pentode and was a workhorse of the VHF TV receiver. The type was introduced in 1950. Much used for RF, intermediate frequency, and video amplification in 405 line television receivers.
The circuit designer is given maximum flexibility by having all electrodes brought to external pins. Many pentodes have g3, the suppressor grid, internally connected to the cathode.
The Denon logo. Inside the outer screen can be seen the the three wire grids.
Here the suppressor grid is clearly a helix whereas the inner grids are shaped to pass flat across the cathode.
The thin glass tube envelope is 20 mm diameter and excluding the B9A base pins stands 60 mm high.
References: Data-sheet & 1040 Type 6BX6 was first introduced in 1950. See also1950 adverts.