Taking the work that's done on helicopters over the years, there's very little of the original left in any of them. XV121 (or any other number) is just the number on the tech. records file.
Example from memory: Whirlwind HCC10 XV763 in for a Minor* - new engine, transmission, transmission platform, main rotor gearbox, front and rear 'A' frames, cabin floor, landing gears, tail cone (with rotor transmission) and tail rotor gearbox. If that's a 'Minor' imagine what a Major might involve.
