I also have a small collection. One is one I found in my grandfather's things. It was a small cheap one, but interesting to me was that it was from an era when

I also have a small collection. One is one I found in my grandfather's things. It was a small cheap one, but interesting to me was that it was from an era when computers just starting to supplant people (he worked on a room-sized computer who's job was to count carpool inventory for the army.) I also have one I got from an army-surplus store in a flea market that has various aerial photography calculations tools on the back.