I had the pleasure of discussing that album with Steve earlier this week. It was their original intent to use only old instruments on that album to play only traditional old songs, but they strayed from that plan. He may have played the Eric Martin on that song, but I believe he plays a Sterling on the rest.
He refers to the Martin as his chromatic C accordion.
The Cajun accordion/one row melodeon with extra buttons is nothing new.
Bouchard in Canada makes 'em, and the possibilities are plenty.
Watch and listen to all the different keys. Good playing , too.