This photo is of the other end of the building and shows the post, but not the top. Without the previous photo, I would have assumed it was just a flag pole. I have looked at several photos of the depots at Tipton and Austin, but so far have not come across one showing similar signal. The photo is from the Baker County Lib. Col.
Here is my theory of operation at Sumpter. First off for those not aware of the track arrangement at Sumpter, the track to Sumpter was essentially a short branch line off of the main taking off at S wye. Adjacent to the Sumpter depot, one leg of the wye headed a little ways up country to some warehouses. The other leg of the wye which I believe was considered the main curved across the street at the depot and made a u turn back along the hill to the mill, which was operated over the years by a number of operators including Stoddard. A feight train unless carrying LCL would have no reason to stop at the depot if it was setting out and picking up cars at the mill or up at the warehouses, so it may be that the signal was used so that a freight would be stopped if new orders were issued which could easily happen in terms of meets with other trains no matter which way it was headed after leaving Sumpter. All a WAG on my part.