Accordionists are well aware of the terms bellows, buttons, and keys but not all accordionists know or care about tone chambers.
A tone chamber changes the timbre (pronounced "TAM-ber") of the instrument and is used to apply tone color to the sound. The most common tone chamber is a wooden box inside the accordion that houses one rank of reed blocks. Sometimes it is a cover that slides into place over the air holes under the grill of an accordion.
The picture to above has examples of two early experiments in tone chambers for the accordion. If you look closely at the accordion picture you can see that early prototypes had the sound box (tone chamber) on the outside of the instrument. One of the additions resembles the bell of a trombone or tuba, the other is a hollow box used as the grill of the accordion. I suppose one could be called a tone chamber and the other a horn, verbiage aside the intent is the same, to change the timbre.
Although I can't say for sure how these instruments sound it is my guess that the open bell would project the sound, like a Stroh violin (see picture to right) and the enclosed box would give a hollow and muted tone.