The distal invagination of maxillary molars can create a bit of a challenge when adapting a matrix band. There are lots of matrix options, but if your normal armamentariam is a toffle-meyer and band, then an application of a little Dycal can help out. Just a dab applied with a perio probe to fill the space. Then smooth carbide to clean up the flash. This can be removed an interproximal carver at the time of amalgam carving or later with a scaler. Having this tip in the bag helps one practice Restoratively Driven Endodontics.