I'd use a vacuum cleaner hose in the open space and gently tap the bellows. One problem is the cloth blackening will have faded substantially, this can cause some flare, so I've used a matt acrylic black paint - Lamp Black - to resatore my Agfa Ansco 10x8's bellows internally. However this needs experience as the sectret it it needs to be just enough to re-blacken without building up a layer of paint on top.