Light,instrument,m390 -what is it?

[Piercesaw]

Picked up a device that looks military. No idea what it is but takes 2 d batteries in the body of it with a switch on top. Has a clamp on it that is wired to the body. Inside the clamp is a hole about the size of a pencil eraser. this is where the light is. Is marked Light, instrument, m390. any idea what it is?
Tia

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[browningautorifle]

Maybe a couple pics would help...

[stencollector]

Light instruments are flashlight devices used to light up the interior reticles or the external adjusting knobs on various artillery fire control system devises like panoramic scopes and elbow scopes. They allow a measure of light discipline in tactical situations at night. I am not sure what a M390 is specifically for, the number sounds quite high for a Instrument light. Any chance the "0" is a "C"?