The acorn valves were designed to have low capacitances and therefore work to higher frequencies than the normal valves.
Radio amateurs used them for work at 144 MHz after WWII.
Thanks to G6RHV we have details of the operating conditions. The information comes from the leaflet packed with the valve and gives the data as 'Tentative Characteristics' and dates from 1936. The year the 956 was introduced. Click the link above for a reproduction of the data sheet.
All three wire grids can be seen through the notch in the anode.
Made in USA.
The acorn envelope is 14 mm in diameter and, including the base pins, is 42 mm tall.
References: Data-sheet. Type 956 was first introduced in 1936.