I've taken some photos of the significant variations on Enfield twin crankcases, along with some primary cases, and parts of timing cases to show how they differ.
I don't have any parts for the earliest model 500 twin engines, but I do have early and late model (pre 1957 and post 1957) engines, and Interceptor engines.
The captions under the photos explain the differences illustrated in each picture.
Click here for pictures.