Part Number:S9574. The hole is slightly tapered so that your motor's shaft will fit tightly. These ones have a slighly tapered 2 mm centre hole. This is important.
HORNBY TRI-ANG WORM GEAR S2375 BRASS SINGLE START WORM GEAR for TURNTABLES
Worm Gear Extension 2 x Worm Gears and 1 x Drive Gear Unused Worms are Glued #1
Worm Gear Extension 1 x Worm Gear and 2 x Bearings Unused #2
Hornby X8004 Worm & Worm Wheel 0-4-0 Gear Set