A cassotto is a 'sound chamber' within some more expensive accordions that contains one or more reed blocks of the treble side of the instrument.
The firm Öllerer built a diatonic instrument with cassotto without register switches and two 8' (M) reed sets.
Making a cassotto for diatonic instruments is quite more difficult due to the different reed plate arrangement.
In traditional German accordion making, the cassotto barely plays a role.
In those models the reed blocks are arranged in one plane like with ordinary accordions.