That's great that they feel it is original! Wayward has a point. If you're tying up your horse, it's usually at a horizontal rail, rather than an upright pole, unless that pole has a ring in it. It could be that a ring was mounted further down on the pole. Your seller said that it was from an old pole at an old barn and that certainly speaks well of it. I would contact them again, and just let them know you want to learn the history of the piece- What was the pole like, where was it located in proximity to the barn or house and was there a second ring.
When you tie off a horse you do it with a slip knot. This is because horses occasionally get spooked and do what's called "pulling back" where they heave their full weight backwards. If you tie a regular knot, you can't free the panicked horse because the knot will tighten. With a slip knot the horses end will hold fast, but you have an end to pull that enables you to free the horse if you need to.
If you tied off to an upright metal post pole, it woud be slippery and the knot would slip to the bottom of the pole. This could easily allow the horse to become entangled in the lead line. That's why hitching rails are horizontal and high up, and hitching posts usually have a ring of some sort, to keep the knot from slipping down. There are exceptions though. In my little ol' logging town, we have a few old hitching rings embedded in the concrete sidewalks. These are low, only giving about 6" or 8" between where the horse is standing at the curb and where the ring is mounted. I suspect these were added during the transition period from horses to cars and that prior to that a more horse-friendly horizontal rail would have been in place. Tying off to a slippery metal upright pole is also a bit more difficult when you tie the slip knot, and it also has a much greater likelihood of loosening up as the horse moves around and the knot is slipping around.
