Looks like a Microfilm reader?
As for the joke... It's like the dungeon was so huge they had to "shrink" it to microfilm to be able to fit in "1 page".

I wouldn't say it's a great joke, but that's what I got out of that.

It's a bit before my time, but I believe this is microfilm. I still see these at the library but I've never had any reason to use one.

The things in the boxes are microfiche, an archival solution used before scanners were common. The images on them are fingernail-sized and require a bulky device (also pictured) to read.

I work with these things. A fiche is the size of my hand and can hold about 10x23 A4 pages, maybe 10x24 if very dense. Or one ginormous dungeon map, if you really wanted.

200*