The CV5377 is a special quality version of the EF91. The M8083 has a more rugged build than the EF91 and was designed for operation in areas of vibration especially aircraft and styled CV4014 for UK government procurement.
The prototype for the CV5377 was the CV4014, and the CV5377 is a long life version with low cathode resistance.
The multiple mica discs confer vibration resistance. The anode is the pair of plates joined by a strap. The bright central part cylinder is a screen.
The blue band that inhibits gas release from the glass is dark and thus thicker than normal, this makes observing the internal structure difficult but the outer grids can just be seen.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 46 mm tall.
References: Data-sheet & 1040. Type CV5377 was first introduced in 1964.