The 6CB6A is a short cut-off high impedance voltage amplifier pentode. This exhibit carries the alternate designation of 6CF6. Internally the glass seems to be coated - probably to avoid electron bombardment of the glass releasing gas molecules.
The etched Type designation on the envelope and the printed RCA logo below. Note the shield covering the anode lead.
The beam plates can be seen to pressed from a single sheet and within the electron stream are just two vertical strips on each side of the cathode.
The envelope is 18 mm in diameter and, excluding the B7G base pins, is 47 mm tall.
Reference: Data-sheet. Type 6CB6A was first introduced in 1957.