20 Pdr training aid or armorers tool?

[motto]

I have a device that is made from the rear six inches or so of a 20 Pdr shell case headstamped RL 1951. I am aware that RL stands for Royal Laboratories. The priming was apparently electric and there is a light bulb mounted where the primer or gain originally was.
There is a 1" section of the rim machined away and the machining continued until the interior of the case is visible through an opening of approximately 1" x 1".
It has a brass cup that fits over the forward (open) end of the case with a finger ring in the middle. The cup is a sufficiently tight fit that the whole device can be carried by the finger ring.
Does someone know what the purpose of this device may be?

David