The 6063 is a special quality version of the commercial U78 miniature HT full wave rectifier. The construction is of two anodes set around a common cathode assembly and the SQ version has double micas top and bottom to prevent early damage due to vibration.
Research into modes of failure of valves subject to vibration, as found in aircraft etc., found that adding a second sheet greatly reduced the destruction of the mica.
The GEC paper logo. Marconi and Osram were the commercial brands of M-OV and GEC (the parent company) the professional brand.
The secondary lettering. The valve is also labeled CV4005.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 60 mm tall.
References: Datasheet & 1040. Type 6063 was first introduced in 1951.