The B7G base was the first miniature base for all glass valves. It was first reported by RCA in April 1940. Battery types 1R5, 1T4, 1S5 and 1S4 were the first Types.
The conversion from the acorn 954 to the B7G based 9001 took place in 1941-3 and the CV138 was first made in 1944/5.
The B7G base was not used in the UK until 1947 and valves such as the EF42 (B8A base) and EF91 were shown by Mullard at the National Radio Exhibition that year.
In 1935 RCA introduced a range of valves with steel envelopes in place of glass and a new eight pin Octal base (IO). The stem and pinch was discarded and a pressed hard glass disc was developed to allow the connections to pass from the vacuum to the outside whilst also sealing the glass to the steel shell. The glass disc was formed from a short length of tube that was softened by gas flames and then compressed around the pins. To avoid stress on the glass seal the external leads were wires that were subsequently soldered into the visible pins in the Bakelite base plate.
The machinery that made the glass disc for the metal valves was adapted and the external wires were replaced with pins for the new miniature valves. Initially the pins were reduced in diameter just under the glass so that and bending force would bend the wire and not damage the vacuum seal.