Re: What kind of accordion did Sean Ardoin use for "Zydeco Happy"?
He's using a Cajun accordion in the key of G.
The 2nd position (pull) in G is D. The song is in Bm, which is the relative minor of D. The notes and playing position stay more or less the same for the pull key and its relative minor, so you might as well say he's playing on the pull.
For the young generation, G is a widely used accordion. When played in the 2nd position (D), it's a very good key for males to sing in a relaxed, soulful style.