The ECC804 Mullard Type designation is printed smaller than the 6/30L2 indicating that the 6/30L2 was an established design also made by Mullard. The valve is a general purpose double triode with a screen between the sections. The main application is in television receivers.
The grid is wound on copper supports and the central screen is left bright.
Seen face-on the cathode tube can be seen as a rectangle and looks to be of the locked-seam type.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 48 mm tall.