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the last two digit, 90, in the HIN is the model year.

1990 is correct model year, not when it was manufactured.

I have a 2008 Jeep that was manufactured in 2007. What year is it?
2008

Just like that 1990 Outrage you just bought.

Familiarize yourself with how Boston Whaler and other boat manufacturers tend to follow the auto industry on how and when a vehicle or Hull is identified.
Whaler uses August as the next years model while autos use September or October these days.
See the "New Year Format" in this article.
http://www.whalercentral.com/articles...icle_id=61

The VIN or HIN tells us everything.
VIN = Vehicle Identification Number
HIN = Hull Identification Number

Enjoy your 1990 Outrage 19'
Excellent model.