The middle picture shows an open pontil on the base of a cylindrical medicine bottle.The third picture shows the base of a milk bottle from just after the trun of the century.the right side is crude and was disturbed by the action of marking the marking with Owens Ring.Owens rapidly made improvements as eh redesigned his machine numerous times and eventually over came the problems of uniformity see here.Note the diagonal line which cuts across the base is obliterated by the Owens Ring (the large off-center curcular feature). the base is crudely made for a machine made bottle.
The age of bottles dating after 1860 can be narrowed down by taking a close look at the lip of the bottle.
All of the bottle mouths shown below were machine made.
In the 1920s, the bottle mimicked early forms which were hand tooled and sealed with a cork.
One approach to helping beginner identify their old bottles involves show them the bases of old bottles.
The picture below at the left shows an iron pontil on the base jof a historical flask circa 1865.