Chain case input and Gearbox output drive spider. There are two different versions of this spider one has an under cut at the entry end of  3mm to seat on the snap ring fitted on the chain case  input shaft Part No 53/4 and measures 18.4mm overall. The Earlier version has no undercut and still measures 18.4mm so this new made part can be fitted on either input shafts Part No 53/4 with or without a snap ring circlip