As found on service sheet 421.
Hornby X1032 Loco Base Bottom Plate With Pickup Contacts For 2021 Class 9F Loco
X6336 Hornby Spare Loco Chassis Bottom *Lower Half Only* for Tornado A1 Class
Hornby OO West country class Battle of Britain loco bottom plate vgc #RR
X9354 Hornby Spare Loco Chassis Bottom + Contacts Pickups for 6800 Grange Class