The drawing here is intended to show the basic linkages of the concertina's valve assembly. The style depicted here is one of the oldest that I have seen in use. As should be obvious, depressing the button lifts the valve pad assembly from the hole in the valve board. This allows air to flow into the chamber in the reed pan below, and through whichever reed is appropriate for the direction of bellows travel. What perhaps is not obvious from the drawing is that each valve pad and valve hole are round.
Accordions commonly have rectangular valve plate assemblies.
Some particularly large concertina reeds may have a pair of overlapping valve holes, and a valve pad which was oversized, but has had two sides cut off to fit. Occasional other valve pads will have a side cut off to allow it to clear the adjacent pads. There are detail differences between the Wheatstone and the lachenal metal buttons. In particular, the Wheatstone is fabricated by pressing a deep-drawn metal cap over a body of wood in the older ones, or plastic in the later ones. In contrast, the Lachenal buttons are all metal.