While Butterworth's patent defined the relationship between the buttons, there have been a large number of layouts over the years, in terms of number of buttons, their range, and their spacings. Here are some examples I have found - more welcome!
From the 1896 Lachenal Tutor
The following four layouts are all taken from the tutor published in 1896 by Lachenal. The centre column of buttons is positioned slightly lower than normal (or were early models made like this?). Middle C has been circled.
A standard 48 button layout
Thanks to Ryan Smith for a neater version of the 48 button layout
A Crabb 80-key layout
This layout, used by Crabb up to the 1970s, was published by Geoff Crabb on concertina.net
A Crabb 69-key layout
Layout diagram produced by Geoff Crabb for the owner of Crabb 9978.
A Crabb 58-key layout
Kindly provided by the owner of Crabb 8981
Button size & spacing - Crabb v Lachenal
Geoff Crabb has kindly allowed the inclusion of this diagram, showing the relative button spacing and sizes on Crabb and Lachenal Crane duets.