I drilled a tiny hole in the centre of each 'A' button on my Hayden and glued in the cut-off head of a brass nail.  This lets me feel where 'A' is and relocate my fingers when they get lost.  Of course, the finger that touches an 'A' does change depending upon the key being played but that does not seem to be a big problem.  I ordered some extra buttons from the Button Box just in case this did not work out, or if I ever wanted to sell the concertina.     FWIW.  These button