The 6X5 is a full wave indirectly heated rectifier. The RCA data gives a high voltage specification but other makes quote 350-0-350 as the input voltage. RCA specify a small reservoir capacitor.
The 6X5 was made with a standard glass envelope (6X5G) and a glass tubular envelope (6X5GT) as well.
Made in the USA, code and 6X5 stamped on the base.
The RCA logo.
The thin metal tube envelope is 25 mm in diameter, and excluding the IO base pins is 68 mm tall.
References: Data-sheet, 1040 & 1043. Type 6X5 was first introduced in 1936. See also1936 adverts.