A locking system for a stop button employed in a mortise lock having a spindle for operating a latch utilizing a first spindle base and a second spindle base which are nested relative to one another. Each base includes an opening for the mortise lock spindle such that the spindle may enter the combined first and second bases from the left side or the right side. A locking hub overrides either the first or second base and is capable of selectively lying over either the first or second base. An abutment linked to the stop button interacts with the locking hub such that either the first or second base may not be turned by the spindle when this occurs. Locking hub selectively locks either the first or second base through its position relative to the same and allows the mortise lock to easily transform from a left-hand version to a right-hand version.