The first markings were added in 1957 and overlapped into 1958.
Slim lighters were first introduced in 1956, with a flat bottom and no date code. This should provide the information to fix a relatively precise date for any Zippo lighter. While the information for dating a Zippo is widely available, it isn't that handy to find it all in a single location.
Reference for Dating Zippo Lighters How old is your Zippo lighter?