The E288CC is a special quality double triode with a minimum life of 10,000 hours. It was designed for use as an Rf or IF amplifier and within the cascode circuit topography.
The anode with the deep depression to the working face.
The double mica discs are there to reduce the effects of vibration.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 52 mm tall.
Reference: Data-sheet. Type E288CC was first introduced in 1968. See also1968 adverts.