The 396A was originally designed for RF applications up to UHF. It is described as a medium μ double triode for VHF/UHF but recently it has been re-purposed as an audio valve in much the same way as the ECC81.
The valve also carries the Type designation 2C51 indicating direct equivalence.
The two small towers with the screen between.
Here the anode can be seen as a small box with a side flange for fixing and extra heat radiating surface.
In this view the construction methods are clearly seen with tabs folded over the mica discs to hold the components in place. The heater seems to be a triple hairpin within the cathode tube.
The thin glass tube envelope is 21 mm in diameter, and excluding the B9A base pins is 37 mm tall.
Reference: Data-sheet. Type 396A was first introduced in 1960.