The 6063 is a special quality miniature HT full wave rectifier. The construction is of two anodes set around a common cathode assembly.
The Brimar paper sticker with a capital T for Trustworthy. Did they want to send the message that their commercial valves were not trustworthy?
The top of the anodes has a pair of mica discs separated by a space. This extra rigidity was found to greatly extend the life of valves subject to shock and vibration especially for aircraft equipment.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 58 mm tall.
References: Data-sheet & 1040. Type 6063 was first introduced in 1951.