Here's an interesting little gimmick, most likely from the 1920s. This one (and I'm sure there are many from different suppliers) was provided by Red Chain Feeds. It's in fair shape and measures 3.5 by 5.5 inches. It was designed to help you pinpoint your live stocks' special days.
It works like this: You set the inside dial to day on which your swine were bred. Then, a powerful magnet goes to work and behold! As you close the cover, it snaps the metal arrow on the front to the date the piglets are due. It's magic!
"After You Breed 'Em Right Then Feed 'Em Right"!