Apparatus for preventing incorrect installation of machinery components together includes a tab, pin and pin-receiving hole. The tab on the seal plate protrudes angularly from its periphery and laterally beyond it so as to prevent incorrect installation of two seal plates together in a faulty sealing relationship with machinery components by their tabs making interfering contact with one another producing a visible gap between the two seal plates. The pin is mounted on and protrudes axially from a surface of one machinery component facing the seal plate and prevents incorrect installation of the machinery components together in a faulty sealing relationship without the seal plate between them by the pin contacting the surface of the other machinery component and producing a gap between the machinery components. The hole extends through the seal plate to receive the pin within it when one seal plate is correctly installed between the machinery components.