The 955 is one of about a dozen types of " acorn valve ", so called because their size and shape is similar to the acorn ( nut of the oak tree ), designed to work in the VHF range.
2.
An "'acorn tube "', or "'acorn valve "', refers to any member of a family of UHF vacuum tubes starting just before World War II . They were named after their resemblance to the acorn, specifically due to the glass cap at one end of the tube that looked similar to the cap on an acorn.