I will be working with Airstreams Archivist/Historian over the coming winter months to extend the 1959-80 Airstream Serial Number Decoder to include 1958
and prior, based upon the incomplete Airstream production records that exist and using our best combined judgement. Until then, I consider both O7415 and O7499 to be 1957
O7415 was built in April 1957 and should be a 13-panel Overlander as except for one known 7-panel prototype (S/N O7373) Ohio did not change to 7-panels until July 1957 near S/N O7463 +/- 10. We don't know the exact 7-panel breakpoint.
O7499 was built at the beginning of Sept 1957, with the change to 1958s coming about 3 weeks later during the week of Sept 23, 1957 starting at serial number O7513. The 1958s were a little wider than the 1957s. The 1958s also got Hehr Clearview front and rear windows, while the 1957s had Hehr Standard front and rear windows . . . at least officially. It is possible that the window change did not occur at the exact changeover to the wider body for 1958.