The disk-like mark is sometimes confused with a pontil.
the pontil is actually broken glass where the metal rod used to hold the bottle while the lip was form was broken off leaving a sharp scar.
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.
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.
(Specifics on what a pontil looks like or how to tell the age based on the mold seam can be found in Bottle Basics.) While these two characteristics are often a strong clue to age, readers will be further helped by developing an understanding how the various categories of bottles changed over time.
To aid beginning collectors and those interested in bottles I have developed a number of bottle time lines.
The pictures below are from two early machine made medicine bottles.These diagrams should help clarify age differences based on both form and function.With each chart the reader will find accompanying pictures to further aid in bottle identification and age.The glass is rough (not sharp) around the circumference of the Owens ring.Notice also how unlike most pontil marks, the Owens ring covers the whole base of this bottle.