[Arran]

I think part of the plan may have been to save on brass but when it comes to octal tubes Philco had a hate on for metal tubes and prefered ST style glass tubes so in certain caes they may have left at least the #1 pin contact out so a metal tube could not be used. Also rememeber that in 1937-38 even metal octal tubes still had a grid cap, the 6S## tubes seem to have debut in 1939 so on a 6K7G two pins were not needed as anything other then a tie point.
I noticed that Rogers in Canada (Also known at the Canadian Radio Corperation and Rogers-Majestic Corp) did similar things when it came to their 2X3 and 2Y3 rectifier tubes. The 2X3s were diodes rectifiers used in pairs where the filaments were connected in series. The basing is similar to a 5Y3 minus a plate, but they only needed three contacts out of eight on each octal socket and often left the other contacts out. The main reason I can think of was to prevent servicemen from rewiring the socket to accept a more normal 5Y3 or 5Y4 .
Regards
Arran