RCA, Philips, Tungsram and Telefunken all seem to have been developing button bases to replace the pinch.
Reasons:
Automation (saving money)
Performance: Shorter leads = less inductance, better RF
Size: The Acorn though proved to be a mistake. Too hard to make with too many rejects.
Here is a rather poor article with some good links that I did.
https://www.radiomuseum.org/forum/fr...to_button.html
I don't think the B7G was solely for battery valves, though RCA's 1940 (radios in 1941 before Pearl Harbour) were a miniaturised version of Sylvania's 1938 first "All Dry" 1.4V valves using 50mA filaments. Telefunken instead optimised for parallel battery filaments in the Y8 low profile metal case with button base. These were used during WWII and into early 1950s, killed off by Philips 25mA types.
So Dutch, Americans and Germans simultaneously doing button bases to reduce manufacturing costs and improve RF performance.
The Dutch Philips EF50 9 pin similar to Sylvania loctal and the Philips B8G 8 pin with pip to replace B7G and Octal on mains and battery valves. DK40 good example vs 1L6.
The EF91 was inevitably to be replaced by a Noval.
The 7 pins and 8 pins wasn't enough. Though there was an B7G triode hexode for mains using one heater pin for cathode and the US sets tended to use Heptodes/Octodes and not worry about valve count.
https://www.radiomuseum.org/forum/ea..._innovals.html
The Noval 9 pin base was inevitable for Europe/UK to avoid top caps, screened RF Pentodes, ECH81, EABC80 etc. Also more power than B7G.
RCA wasn't concerned with valve count or Triode-Hexodes, hence 7 pins on B7G rather than 8 or 9. Also allowed a smaller package for the military gear; shell & bomb fuzes, communications, walkie-talkies. The battery valve personal sets was an obvious publicity for the miniature tubes.
https://www.radiomuseum.org/forum/co...es_valves.html
Very many sets before USA entered WWII. But certainly one goal was smaller, higher performance military gear.
https://www.radiomuseum.org/forum/rc...article.html#3