HiHo (if I may be so informal as to address you by your first name), if you can find out when the accordion was made, that could provide a clue as to what's going on. An international conference in 1939 established Standard or Concert Pitch as A=440 hertz (cycles per second). Prior to that it had been all over the map, as low as 423 and as high as 455.

I'm just guessing, but that may have something to do with it.

Don Firth