How do you date old Kerr Mason jars?

Check the jar from top to bottom. The Kerr name will be embossed on the glass. There are some general guidelines to approximate the date of a vintage Kerr jar between 1915 and 1930. The finish on a fruit jar can tell you roughly about the date it was made.

What are the bumps on the bottom of jars for?

In short, these bumps, raised dots, or small divot are to help notate which mold and machine were used to make the bottle. Should a defect or problem be found on a bottle, the manufacturing record can be tracked and all other bottles made from or within the batch can be easily inspected.

What are the bumps on the bottom of bottles for?

These are typically called “mold codes” or “heel codes” and there are many different ideas about what these are for, but I could only confirm one. It is a popular notion that these bumps help the bottler know how many times the bottle has been reused.