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Yes, an 8A would be the identifier for a 20' Whaler
http://www.whalercentral.com/articles...icle_id=22

So if you have 3A7428 that would make it a 1973 model of some sort and manufactured in May, 1973.
http://www.whalercentral.com/articles...cle_id=102

We have many Stencil and HIN numbers of models here for future reference.
http://www.whalercentral.com/articles...p?cat_id=7

Do you know the model or is it a Custom?