I dont actually think it has a name ,Sometimes it is part of the ribs be it machined or brazed on and filed but frequently it is just a piece of steel filed to fit and soldered into place .
Names could be fillet , plug , wedge as in double rifles.
If it is only a small space then the common thing was to file a close fitting triangular piece about 1/2" long with a slight taper so when tapped into place prior soldering it fits as close as possible and is therefore hardly noticeable .