Airstream's pre-1980 VINs are here but they don't seem to apply to your case - http://www.airforums.com/forums/atta...3&d=1189129122
Closer to your particulars is R.J.Dial's vintageairstream.com. Specifically - 58Overlander
At the upper left corner for Ohio-produced coaches it gives the VIN example as "Ohio | O-7550 to O-7741 (I prefix for Int'l options)."
An International optioned Overlander could have a VIN on the order of I-7642. Which falls between the sequential production numbers of 7550 and 7741 noted at RJ Dial's site. And your VIN badge shows OI-7642.
For 1957s at R.J. Dial's site, it gives a maximum production number of 7593. I'd say you have a '58 Overlander, International package. The pictures sure are a match!
note the folders along the left side starting about 4" from the top -- those represent photo folders of seven different '58 Overlanders. Too bad none show the VIN badge up close.
Congratulations on your classic!