Not sure, but he has many different brands...a whole closet full. Jr. Martin was not making accordions at the time of "Cajun Life." I'm pretty sure it is not a Master, although I know that later he recorded "Tear Stained Letter" for RCA with a Master Bb. Magnolia sounds like a good guess; I remember seeing him on Passe Partout morning shows and it seems like it was a blue accordion...hmm... I am pretty sure he used a Monarch or Sterling on "Chere Alice" and and anything else in C or G. Flemmes was on a D and Allons a Lafayette, Perrodin, Bayou Teche, Jambalaya, and maybe a few other were all on an F accordion.
I had the pleasure of working with Joel a couple of times ( running sound ). I asked him about the sound of his accordions on his different recordings and he told me that the sound is produced in the studio with effects ,in an effort to reproduce the "old time" sound.
So , I believe the answer to your question is really a mute point. It doesn't really matter which brand accordion he used. The sound was produced by the magic of the studio electronics.
Jude