On this parts fiche the O-ring(61a) is the most specified component.
Parts such as the Repair Kit 1: Gasket, Cover (nas), Oil Seal(61a), O-ring(61a), Gasket, Manifold 1 (nas) are included in the L250trp 1991 Repair Kit 1 parts fiche.
The sub assemblies and components that make up this Repair Kit 1 include this Gasket Kit (nas), the most expensive part.