A good used Jaguar all synchro gearbox reverse gear striking rod plunger & detent ball, in the manuals it's listed as C13334 (items 56 & 55 in the illustration) and fits quite a few models with the all synchro gearbox but obviously check compatibility before committing to purchase.

This came off a MK2 gearbox serial number KCN532 but will fit the S Type, 420, MKX and the E Types that were fitted with all synchro boxes.