Bill, is there a diagonal line across the base? Its hard to tell in the pictures... If so, and what I'm seeing is not just a play of light, then it looks like you have a diagonal hinge mold or snap case, or possibly key mold because it sorta of looks like the mold line "pin wheels" over to the corners...... As for the dot, its pretty much just there to throw you off! Could be a deformed dimple from a vent or some type of alteration or repair. Usually when you have a mark like that its better to disregard it in consideration of the mold.
If I'm just making it all up and there are not any mold lines across the base, then you should find a mold line running all the way around the base, about 1/8 or 3/16 of an inch up the bottle, making it a cup mold...
Hey all
No seam on the botton that I can see .The pic really has no distortion thats the way the bottom really looks Gunther .Its rough and has a bit of a bulge to it in the center making the bottle rock a bit . Heres a pic looking at it in the light .And yes it's the Zanesville flask .