Treewind is right about the flattening under pressure ... and it being worst in the low notes. I was listening to recordings I made of theRev. Alex Richards, back in the '80s - playing in his lovely "stage virtuoso" style on very nice, high-grade Wheatstone concertinas (actually Æolas ... their 8-side top models). Despite recent, and careful, re-tunings there still was a tendency for the big final chords to sound flat if he pushed the volume. I had to suggest that he work in "microphone" terms and lay off the pressure - moving a little closer to the mic. for the final flourishes.
treewind: That Lachenal was probably, originally, tuned to "Kneller Hall" pitch - standard Brass Band pitch of the day. A = 456 is pretty close to a ¼-tone. Most woodwinds and other band instruments of that period would have been at the same pitch, and the brass-reeded concertina would have played happily with flutes &c - without having to push hard ... and sound flat.