(!!!) 231=31 US Made Balloon NOS 1920's (14.2ma)
(!!!) 31 RCA Radiotron Balloon NOS 1920's (13.2ma)
(!!) (1 Pair) 31 Cunningham NOS 1940's (11ma and 11.6ma)
(!) (1 Pair) 31 Sylvania rebranded Zenith NOS 1940's (12.8ma and 13.6ma)
(!) 31 Ken Rad NOS engraved base 1940 era (11ma)
(!) 31 Tungsol NOS 1940's (11.8ma)
(1 Pair) 31 RCA/Cunningham NOS 1930's - 1940's (12.0ma and 13.2ma) (Same Gm)
31 Philco by Sylvania engraved base NOS 1930's (11.5ma Gm=1000)
