As found on service sheet 459 for R3989 & R3990.
Hornby X454 Cylinder Block For Early Britannia Class 7MT Loco
HORNBY X820 CYLINDER BLOCK FOR R859 BLACK 5
TRIANG HORNBY X454 CYLINDER BLOCK FOR EARLY BRITANNIA CLASS 7MT LOCO
Hornby X7957 Cylinder Block & Valve Gear For Class P2 R3984 & R30350SS
Hornby X6669 Cylinder Block Assembly For Class 4000 Star R3229 British Monarch