"definitely in meantone tuning."
Well, I haven't had the sounds professionally analysed, and a couple of notes are out of tune, but all the G#s are in tune with each other, all the Ab s are in tune with each other, all the D# s are in tune with each other and all the Eb s are in tune with each other; but the D#s are about a quarter-tone flat of the Eb s and the G#s are about a quarter-tone flat of the Ab s. (It was when I noticed this that I started reading up on meantone tuning). A scale of A major or Eb major sounds noticably wrong if you play the alternative accidentals.
The thirds sound purer than the fifths. But I noticed this after reading up on meantone temperament, so this could simply be my ear telling me to believe what I read.
The pitch, by the way, is around the Society of Arts pitch of C=530, although the concertina is around 1851 in date.
I wouldn't want to have the instrument tuned to modern concert pitch and even temperament - it would seem wrong. I think I'd rather play out of tune with everyone else.
(thanks to everyone who has replied to my original message)