Yesterday, I adjusted the key travel of our Bb/F Jeffries concertina.
According to "The Concertina Maintenance Manual," the key vertical travel should be around 3.0 to 3.5 mm. But, its key travel of the right-hand side was more than 5 mm.
I added one more felt washer (button damper) to every key.
It took about half an hour for me to add twenty felt washers. Now, it's easier for me to play than before.