The T41 is an early thyratron and this exhibit is unused. The 6K25 is identical except for its 6.3 V heater.
The thyratron is a special gas filled valve that at a given grid voltage rapidly moves into full conduction, when the gas ionises, thus acting as a switch. Under the correct circuit conditions this action can be made to generate ramps or pulses.
These valves were used in early radar and television as ramp generators, and in the first computers as pulse generators.
A typical thyratron TV timebase circuit of the early 1950s.
The classic envelope is 32 mm in diameter and, excluding the MO base pins, is 72 mm tall.
References: Datasheet, 1040 & 1043. Type T41 was first introduced in 1940.