I'm writing a play right now where there are several sailor characters. The setting is supposed to be up in the air so whoever stages it can do whatever, with it generally being sometime between 1945 and the present. I've watched a lot of MASH in my life, and I vaguely remember some characters having a small device resembling a jeweler's loupe, but with a closed end instead of a lens, containing a small pin-up poster or similar piece of visual erotica that would be magnified by looking into it. A character having one of these is relevant to the plot (zombies are involved). Were these a thing, and if so what are these specifically called (colloquialisms are fine) and were they popular?
I don't remember that episode.
Could it be a slide viewer like this? OR like this one
The way they work is that they have a translucent back, a slot where you insert the slide, and a lens. You hold the frosted end up to a light source and the light shines through the slide and is enlarged by the lens. Other models have a built in light bulb. You would use this to sort your slides for a slide show.
Some of them have a transverse slot so that you could view negatives without cutting and mounting them.
Perhaps /r/photography could tell you more.