The EPF80 is probably a secondary emission pentode. Sadly we have not found any operating details as yet.
See EFP60 and EE50 for other examples.
The screen obscures most details. Some wire grids are visible but not enough detail to determine the functions.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 59 mm tall.
Updated April 17, 2013.