It's most probably a 955
http://www.r-type.org/exhib/aaa0110.htm
A VHF triode somewhat similar to half of a 12AT7/ECC81. It's raison d'etre is the very short connections to the electrodes offering little series inductance.
954 is the pentode of the family and has pins top and bottom in the glass pinches. as well as the ring of pins the 955 has.
Plugging and unplugging these valves was very difficult due to stressing the seals.
The socket looks like a white pottery ring with clips bolted to it for the pins. It converts a tiny valve into something quite space-hungry.
David