Some players are put off the Castagnari 4-stop one-row by the position of the air button right at the top of the growl box. It is operated by the tip of the thumb and can be difficult to reach for players with small hands.
Here is a modified air button installed in a new Castagnari Mélodéon. The position of the old air button is marked by the domed plug used to close the hole it occupied.
This is version 1.0 and there may be improvements once the new owner has played it for a while.