It's not unusual to find "I" and "1" used interchangeably in engine serial numbers. Generally, unless otherwise noted, if you see an "I" in an automotive serial number, it is considered a "1". Same goes for "O" and "0".
The code you have posted can be read two different ways, depending on what the casting date next to the starter is. If it is a '57, as you have originally suspected, it is a Flint built Passenger 235. assembled on Jan. 15. But if it's a '59, as Dave suspects, and I agree with from what I see, the number works out as the same plant/application, but the date assembled could be May 11, as '58-'62 went with a day/month format instead of the month/day format found on the earlier engines.
