Kerr self-sealing jars and the Kerr Economy were created in 1903. Alexander H. Kerr also founded the Hermetic Fruit Jar Company in the same year. San Francisco man Julius Landsberger patented metal lids fastened onto a composition gasket. Kerr used the patents to create a revelation in preserving food. Kerr, in 1915, created a flat metal disc lid to fit onto a Mason jar to use with other canning jars.
Inspect the jar from bottom to top. Look for the Kerr name, which is embossed onto the surface. Observe the finish of the jar. If the base is smooth at the jar's lip, it was made by a machine after 1915. A rough base means the jar was made between 1900 and 1930.
Look for mold seams. Jars not made by machines were free blown; no mold was used. A number is usually embossed on the jar's bottom, dubbed a "mold number." This will refer to the blower and his team. A mold seam means the jar is post 1915.
Look for small scars at the bottom and look to see if the design is more modern. This means it would be post-1930, during days of more efficient machines.
Look for a date of manufacture, job number, plant location and various other information. This would mean the jar is modern.
Only use a Kerr jar lid once before disposing.