ORIGINAL: GuntherHess
That is one of the types that is hard to tell on. Its not a tradition key mold where you just have a semicircle or half rectangle.
It looks like it may be a mold with the circular post afixed to one half of the mold.
You can see more excess glass on the bottom side of the circle where the mold parted. Dont know if the top half was afixed or just fit tighter. In either case it apparently did have a post so its fair to call it a post mold.