These fine charts were prepared by Nick Robertshaw, a Jeffries duet playing British ex-patriot in the USA, who offered them to me to form part of the Concertina FAQ. Like Marc Lamb's charts they are particularly clear. Nick says:
This is for an instrument in C. The Jeffries duets are 'centered' on a particular key--look for the major triad (C E G) on three adjacent buttons on the second row down. I also have a D box (one tone higher). The larger instruments extend the right hand down to increase the overlap and have more notes in the lowest left-hand octave.