My guess, based on the shape and the square hole, is a mounting block for an elliptical leaf spring from a buggy, carriage, wagon seat, ect. At first I thought regular leaf spring mount but the axle slot and leaf spring slot would normally be crossways instead of running the same direction. But with elliptical springs the spring is sideways so both slots are inline with each other.
That is just a guess and if that's what it is it would be pretty small or just a spring mounted seat type.
http://www.initialdave.com/cars/tech/suspensionbasics03/ellipticsbig.jpg
Below is a leaf spring style(modern example pictured) as opposed to elliptical spring.
http://static.speedwaymotors.com/RS/SR/Product/91639036_R.jpg
Since it looks to have a date on it, I am skeptical about my guess. From the pictures it looks pretty small, how big is it? What is it made of? Is it magnetic?