Codes were not used on the lighters until 1987, so we have to look closely at the bottom stamp and the insert to try to determine the date of manufacture.It is not unknown for Lighters from Canada to to be shipped with Bradford stamped inserts, so this can sometimes be a help.For this reason this page should be considered as a guideline only.
On the two lighters below we can see how the spacing between the letters differed over time.
On the first lighter, ZIPPO starts between the 3 & 2, whereas on the second it starts between the 2 & 6.
Some important points to note are the shape of the "3" within this patent number, some lighters have a round top 3 whilst other have a flat top.
Also look carefully at the placement of the word "ZIPPO".
According to the dating chart, the two advertising lighters - Allied Van Lines & a gas company, were made in 1970. The military lighter (3525 Aircraft Gunnery Squadron) was made between 1937-1950. All lighters have been used and have surface scratches.