I just got this oddball device, which is an Ilex shutter with no optical elements, just a flat piece of glass at the front to keep your fingers out of the shutter. The back of the shutter is fitted with a large tube and a clamping screw, evidently to slide over some projection lens.

What was this for? My best guess is for some form of flash training, where an image was projected for a moment and you were supposed to be able to identify what the image had been. Something like aircraft identification training.

I'll ask Les to post some photos.

My understanding is these things were used during the war to train pilots to instantly recognize enemy aircraft. It was placed on the lens of a slide or film strip projector.
I guess if you saw the thing flash a picture enough times, the images would stick to your brain. I have one I was using as a front mounted shutter but it's worn out and I don't think I can fix it any more.

When I was in high school I took a speed reading course that used a device called a tachistoscope to project letters, words, and short sentences for a very short period of time. It was basically a film strip projector (remember them?) with a shutter in front of its lens. This is basically Glenn's application.