It appears to be a "gun camera" training device. By replacing a machine gun with this camera, you can tell whether you really had somebody "in one's sights," during a training exercise. Judging from the lack of sights, it replaces an unsyncronized, over-the-wing fixed gun.

I'm going with the lumber marker explanation. I'm guessing that you would roll this along the end of a squared off log. then when you cut it into boards, they're sequentially numbered. This is important in fine woodworking so that you can match the grain by using sequential boards with alternate side up. As if you've unfolded the tree accordian-fold style.

For holding open a book? Possibly a Bible at a lecturn.

For wrapping insulation around wire?