The DM160 is a miniature voltage indicator. The control range on the grid is 0 to -3.0 Volts.
The other side of the envelope shows the construction. When conducting the DM160 will give out light and goes dark at cut-off. In this way it can be used to indicate the logic state of the circuit it connects to.
Peter Onion says of the Elliott 803B at the National Museum of Computing: The 803 was introduced in 1960 and used six DM160's in the console. The console can be seen in a YouTube video of the 803 running its Algol 60 compiler.
Chay Donohoe says that the DM160 was used in the Marconi TAC computer. The image is of the console of the machine at Bletchley Park in TNMoC. (The National Museum of Computing).
Mullard describe the DM160 as: a special quality directly heated subminiature voltage indicator for use in industrial equipment such as transistorised computers.