I agree with Adam and Per. This is an aftermarket, non-Buick molding. As Per has noted, this was common addition by the Wallace Buick Co. of Portland, Oregon, but I have also noticed that this was a popular item among cars originally delivered in Minnesota and Wisconsin. I suspect that certain dealers promoted these moldings, because the profit margin was higher for them than if they simply ordered the cars with the factory-supplied protective body side moldings.
That's a beautiful Centurion convertible, Tom, and I join other Centurion owners in welcoming you here!
Brian Laurance, BCA # 5168
A Centurion driver since December, 1970
1971 Centurion Formal Coupe