Just a guess, but I suspect that it's something to tie a horse's rein to so the horse doesn't wander off. The base would be for use when the grass is wet and one weight would just slide, or when the ground is Muddy enough to be slick enough to allow one weight to slide, or if you needed a heavy anchor because you were working with a big plow horse that was quite powerful, but it would still allow the weight in the center to provide the anchoring of the horse to the area and I suspect that it's set up to allow the horse to be able to move around and actually turn while the horse moves around so that the horse could eat grass if you were in a situation where it was available. Why would you want two weights instead of one , because each weight is easier to lift then one that weigh twice as much, and in some situations one weight would be enough. If I'm correct these were mostly used with a carriage that the horse pulled so that you just kept the weights in the carriage when you weren't using them. This is all just a guess I don't have any actual facts or experience to prove it but I do know that they used weights sometimes to prevent a horse from wandering away back in the days before the Horseless Carriage.
Maybe the Amish members of Bob is the oil guy could chime in, LOL.
